Silurian Sentence Examples
The oldest rocks of Elba consist of schist and serpentine which in the eastern part of the island are overlaid by beds containing Silurian and Devonian fossils.
The Silurian system was marked by the retreat of the sea from central Australia; but the sea still covered a band across Victoria, from the coast to the Murray basin, passing to the east of Melbourne.
This Silurian sea was less extensive than the Ordovician in Victoria; but it appears to have been wider in New South Wales and in Queensland.
The best Silurian sequence is in New South Wales.
They became extinct in Palaeozoic times, and are chiefly found in the Upper Silurian, though extending upwards as far as the Carboniferous.Advertisement
Cambrian, Ordovician and Silurian beds have been recognized, the Upper Cambrian consisting of a limestone which is very rich in metalliferous ores (especially galena and calamine).
In the western Sierras, which are more or less closely attached to the main chain of the Cordillera, Cambrian and Silurian fossils have been found at several places.
Silurian rocks are well developed in western Tasmania, and the Silurian sea must have washed the south-western corner of the continent, if the rocks of the Stirling Range be rightly identified as of this age.
The lodes occur in Silurian metamorphic micaceous schists, intruded by granite, porphyry and diorite, and traversed by numerous quartz reefs, some of which are gold-bearing.
Phosphatic beds, supposed to have had a coprolitic origin, are found in the Lower Silurian rocks of Canada.Advertisement
The Ordovician and Silurian systems are widely developed, and it is most probable that, with the exception of the Archean continents of Finland and the S, the sea covered the whole of Russia.
The Falkland Islands consist entirely, so far as is known, of the older Palaeozoic rocks, Lower Devonian or Upper Silurian, slightly metamorphosed and a good deal crumpled and distorted, in the low grounds clay slate and soft sandstone, and on the ridges hardened sandstone passing into the conspicuous white quartzites.
Genus includes several hundred extinct species ranging from the Silurian to the Tertiary.
The Silurian mica-schists of Bergen in Norway are fossiliferous; in the Alps it is believed that even Mesozoic rocks pass laterally into mica-schists and talc-schists.
Several Silurian fossils have been identified as insects, including a Thysanuran from North America, but upon these considerable doubt has been cast.Advertisement
It is believed that insects of this group are represented among Silurian fossils.
It should be pointed out that the deposits which have been hitherto of chief commercial importance occur in the old rocks (Carboniferous to Silurian) on the one hand, and in the comparatively new Tertiary formations on the other, the intermediate periods yielding but little or at any rate far less abundantly.
Upper Silurian, having a strong relation to the Wenlock group of Britain, but with an American facies, and Lower Silurian, with a succession much the same as in British North America, are found on the shores of Smith Sound, and Nathorst has discovered them in King Oscar Fjord, but not as yet so far south as the Danish possessions.
Brachiopods first appear in the Lower Cambrian, and reached their highest development in the Silurian, from which upwards of 2000 species are known, and were nearly as numerous in the Devonian period; at present they are represented by some 140 recent species.
The Trilobites are known only as fossils, mostly Silurian and prae-Silurian; a few are found in Carboniferous and Permian strata.Advertisement
The Silurian scorpion Palaeophonus, differs, so far as obvious points are concerned, from a modern scorpion only in the thickness of its legs and in their terminating in strong spike-like joints, instead of being slight and provided with a pair of terminal claws.
It is probable that the Silurian scorpion was an aquatic animal, and that its respiratory lamellae were still projecting from the surface of the body to serve as branchiae.
The crystalline rocks are succeeded by beds which have been referred to the Cambrian and Silurian systems. In the valley of the Trombetas, one of the northern tributaries of the Amazon, fossils have been found which indicate either the top of the Ordovician or the bottom of the Silurian.
In the Maecuru, another northern affluent, graptolites of Ordovician age have been discovered, and Silurian fossils are said to have been found in the Maraca.
Elsewhere the identification of the Silurian and older systems does not rest on palaeontological evidence.Advertisement
The Eastern Andes is a magnificent range in the southern part of Peru, of Silurian formation, with talcose and clay slates, many quartz veins and eruptions of granitic rocks.
Mr Forbes says that the peaks of Illampu (21,709 ft.) and Illimani (21,014 ft.) in Bolivia are Silurian and fossiliferous to their summits.
From the geological formation here the name Trenton is applied to the upper series of the Ordovician (or Lower Silurian) system, and, particularly, to the lowest stage of this series.
Overlying these amongst the Palaeozoic rocks, we meet in many parts of Japan with slates and other rocks possibly of Cambrian or Silurian age.
A hummocky irregular country spreads southward, where the Silurian axis is encountered, in continuation of the southern uplands of Scotland.
The Silurian formation which covers Esthonia, appears in the northern part of Livonia, the remainder of the province consisting of Devonian strata.
In the Lower Silurian formation at Plattsburg and Chazy, in Clinton county, are two beautiful grey or grey and pink marbles, one of which is a favourite among domestic marbles for mantels, table tops and other interior decorations.
The Silurian system is represented by the Baton river beds to the west of the Aorere beds, occurring in the basin of the Motueka river, which flows into Tasman Bay.
Its north-east portion consists of Upper Silurian coral limestones (Llandovery division), containing a rich fossil fauna and representing a series of folds running north-northwest.
The same Silurian deposits are widely spread on the mainland as far as Olenek.
Upon the folded slates and schists which constitute these inliers the Devonian rests with marked unconformity; but north of the ridge of Condroz Ordovician and Silurian beds make their appearance.
Slates belong mostly to the older geological systems, being commonest in Pre-Cambrian, Cambrian and Silurian districts, though they may be found of Carboniferous or even of Tertiary age, where mountain-building processes have folded and compressed these more recent formations.
Shaler, state geologist in 1873-1880, " When the rocks whence they flow were formed on the Silurian sea-floors, a good deal of the sea-water was imprisoned in the strata, between the grains of sand or mud and in the cavities of the shells that make up a large part of these rocks.
While establishing the historic divisions of the Silurian in Bohemia, Barrande also propounded his famous theory of " colonies," by which he attempted to explain the aberrant occurrence of strata containing animals of a more advanced stage among strata containing earlier and more primitive faunas; his assumption was that the second fauna had migrated from an unknown neighbouring region.
Recent discoveries of vertebrates are of the same significance, the most primitive fishes being traced to the Ordovician or base of the Silurian, 2 which proves that we shall discover more 2 Professor Bashford Dean doubts the fish characters of these Ordovic Rocky Mountain forms. Frech admits their fish character but considers the rocks infaulted Devonic.
Silurian and Devonian fossils have been reported at one or two localities, but for the present the observations are open to doubt.
Ordovician System.The succeeding Ordovician (Lower Silurian) system of rocks is closely connected with the Cambrian, geographically, stratigraphically and faunally.
There is in some places an unconformity between the Richmond beds (or their equivalent) and underlying formations, and this unconformity, together with certain palaeontological considerations, has raised the question whether the uppermost part of the system, as outlined above, should not be classed as Silurian (Upper Silurian).
The life, like that of the later Cambrian, was singularly cosmopolitan, being in contrast with the provincial character of the life of the earlier Cambrian and of the early (Upper) Silurian which followed.
Siluriaii System.The Silurian system is much less widely distributed than the Ordovician.
As a result of this emergence the stratigraphic break between the Ordovician and the Silurian is one of the greatest in the whole Palaeozoic group.
In the Great Plains and farther west the Silurian has little known representation.
Either this part of the continent was largely land at this time, or the Silurian formations here have been worn away or remain undifferentiated.
Rocks of Silurian age, however, are known at some points in Arizona, Nevada and southern California.
There are numerous outliers of the Silurian north of the United States, even tip to the Arctic regions.
Both the earlier and the later parts of the Silurian period seem to have been times when physical conditions were such as to favor the development of provincial faunas, while during the more widespread submergence of the middle Silurian the fauna was more cosmopolitan.
As a whole, the system is more-widespread than the Silurian, though not so widespread as the Ordovician.
As in the case of the Ordovician and the Silurian, the New York section has become a standard with which the system in other parts of the country is commonly compared.
This older classification, which has little support except that which is traditional, is still adhered to by many geologists; hut the fact seems to be that the system is set off from the Pennsylvanian (Upper Carboniferous) more sharply than the Cambrian is from the Ordoviciao, the Silurian from the Devonian, or the Devonian from the Mississippian.
The province of New Brunswick exhibits approximately parallel but subordinate ridges, with wide intervening areas of nearly flat Silurian and Carboniferous rocks.
This interior plain of the continent represents the area of the ancient sea by which it was occupied in Mesozoic times, with a more ancient margin towards the north-west against the Archean, where undisturbed limestones and other rocks of the Silurian and Devonian rest upon the downward slope of the Laurentian Shield.
Modiolopsidae.-Extinct; Silurian to Cretaceous; adductor muscles sub-equal.
Nephin Beg, Nephin and Croagh Patrick are typical quartzite summits, the last named belonging possibly to a Silurian horizon but rising from a metamorphosed area on the south side of Clew Bay.
The Muilrea and Ben Gorm range, bounding the fine fjord of Killary Harbour, is formed of terraced Silurian rocks, from Bala to Ludlow age.
Silurian rocks, with Old Red Sandstone over them, come out at the west end of the Curlew range at Ballaghaderreen.
It is formed chiefly of slates, grits and porphyries of the Cambrian and Silurian systems. It consists of five "ribs" converging at the summit, 3560 ft.
The age of a great part of the Palaeozoic belts is somewhat uncertain, but Permian, Carboniferous, Devonian and Silurian fossils have been found in various parts of the chain, and it is not unlikely that even the Cambrian may be represented.
He described also the graptolites of the local Silurian strata; and the.
The Ordovician and Silurian are absent here, and the Devonian rests unconformably upon the Cambrian; but along the northern margin of the Palaeozoic area, Ordovician and Silurian rocks appear, and beds of similar age are also exposed farther north where the rivers have cut through the overlying Tertiary deposits.
The Cambrian, for example, is exposed at Leimitz near Hof in the Frankenwald, and the important coal-field of the Saar lies on the southern side of the Hunsruck, while Ordovician and Silurian beds have been found in several localities.
The south-western part is naturally divided into two tracts by the Niagara escarpment, a line of cliffs capped by hard Silurian limestones, running from Queenston Heights near the falls of Niagara west to the head of Lake Ontario near Hamilton, and then northwest to the Bruce Peninsula on Georgian Bay.
He also described the fossil plants of the Silurian, Devonian and Carboniferous rocks of Canada for the Geological Survey of that country (1871-1873).
There the boulder clay lies immediately on the primitive rock, except in the south-western corner of the island, where a series of strata appear belonging to the Cambrian, Silurian, Jurassic and Cretaceous formations, the true Coal formation, &c., being absent.
In New York it occurs in the Salina beds of the Onondaga series, of Silurian age; and Silurian salt is found also in parts of Michigan and in Ontario, Canada.
It was doubtless this expectation that has led to the description of a number of Silurian and Devonian remains as algae upon what is now regarded as inadequate evidence.
The geologic record is, as perhaps is to be expected, exceedingly poor, except as regards the calcareous Siphonales, which are well represented at various horizons, from the Silurian to the Tertiary; even the Diatomaceae, which are found in great quantities in the Tertiary deposits, do not occur at all earlier than the chalk.
The high Silurian crag now known as Domberg was early occupied by an Esthonian fort, Lindanissa.
The Silurian greywackes and shales that underlie almost the whole of the Uplands weather generally into small angular debris, and at a tolerably uniform rate of disintegration.
In that region the Silurian rocks have been invaded by large bosses of granite and have undergone a variable amount of metamorphism which has in some places altered them into hard crystalline schists.
The highest summit in the south of Scotland - Merrick (2764 ft.) - consists of Silurian strata much altered by proximity to the granite, while the rest of the more prominent heights (all in Kirkcudbrightshire) - Rinns of Kells (2668 ft.), Cairnsmuir of Carsphairn (2612), and Cairnsmore of Fleet (2331) - are formed of granite.
Archean gneiss, Cambrian sandstone, Silurian quartzite, limestone and schist, Jurassic sandstone and limestone, Cretaceous sandstone, and Tertiary basalts, gabbros, and granitic rocks all enter into the composition of the islands.
The Upper Silurian shales and sandstones appear only along the northern and southern margins.
This fossil-bearing set of strata he first styled the "fossiliferous greywacke series," changing it in 1835 to "Silurian system."
Murchison published his Silurian system in 1839, wherein he recognized the Cambrian to include the barren slates and grits of Harlech, Llanberis and the Long Mynd.
So far, the two workers had been in agreement; but in his presidential address to the Geological Society of London in 1842 Murchison stated his opinion that the Cambrian contained no fossils that differed from those of the Lower Silurian.
Salter, the palaeontologist; and in 1852 he included the Llandeilo and Bala beds (Silurian) in the Upper Cambrian.
Meanwhile in Europe and America fossils were being collected from similar rocks which were classed as Silurian, and the use of "Cambrian" was almost discarded, because, following Murchison, it was taken to apply only to a group of rocks without a characteristic fauna and therefore impossible to recognize.
Most of the Cambrian rocks were coloured as Silurian on the British official geological maps.
Nevertheless, from 1851 to 1855, Sedgwick, in his writings on the British palaeozoic deposits, insisted on the independence of the Cambrian system, and though Murchison had pushed his Silurian system downward in the series of rocks, Sedgwick adhered to the original grouping of his Cambrian system, and even proposed to limit the Silurian to the Ludlow and Wenlock beds with the May Hill Sandstone at the base.
This attitude he maintained until the year of his death (1873), when there appeared his introduction to Salter's Catalogue of Cambrian and Silurian Fossils.
Hicks to prove that the rocks below the undoubted lower Silurian of Murchison did indeed possess a characteristic fauna, and their work was confirmed by researches going on in other countries.
On the Norwegian side the Cambrian is perhaps represented by the Roros schists which lie at the base of a great series of crystalline schists, the probable equivalent of Ordovician and Silurian rocks.
The Luray cavern does not date beyond the Tertiary period, though carved from the Silurian limestone.
Cambrian, Silurian, Devonian and Carboniferous deposits are found on the coasts of the Gulf of Finland and Lake Ladoga, and also along the coasts of the Arctic Ocean (probably Devonian), and in the Kjolen.
It was entirely covered with the bottom moraine of the great ice-sheet of the Glacial Epoch, resting upon Silurian sandstones and limestones.
The elevation of the central lowlands seldom exceeds 300 ft., but a few isolated heights of Silurian rock appear, such as Kinnekulle, rising 988 ft.
The Silurian has in Sweden almost the same character as the Wenlock and Ludlow formation of England and consists partly of graptolite shales, partly of limestones and sandstones.
In the western and northern alpine part of Sweden, near the boundaries of Norway, the Silurian strata are covered by crystalline rocks, mica schists, quartzites, &c., of an enormous thickness, which have been brought into their present positions upon a thrust-plane.
The glacial clay of the Silurian regions is generally rich in lime and is thus a marl of great fertility.
Supposed Silurian beds have been described at Portalegre, and in the same neighbourhood Devonian fossils have been found.
The primary rocks which appear at Mitushev Kamen are overlaid with thick beds of quartzites and clayslates containing sulphide of iron, with subordinate layers of talc or mica slate, and thinner beds of fossiliferous limestone, Silurian or Devonian.
The rocks of the Ordovician system, though widely distributed, have not always been separated from the Silurian rocks, which they often closely resemble lithologically.
The extent of the Ordovician will probably be increased by addition of areas, which cannot yet be separated from the Silurian.
The Silurian system is the best-known constituent of the Lower Palaeozoic foundation of New South Wales.
The typical Silurian rocks are richly fossiliferous, the shales containing trilobites, the sandstones many brachiopods, and the limestones a rich coral and bryozoan fauna.
The slates of the Silurian have been bent into folds, and saddle reefs occur along the axis of the folds, as at Hargraves.
Numerous quartz reefs occur both in the Silurian and Ordovician rocks.
Some schists, attributed to the Silurian, but possibly older, contain platinum; and associated with the limestones are beds of copper.
The rocks of the Devonian system rest unconformably upon the Silurian; but some beds of which the age is still uncertain are called Devono-Silurian.
The gold-quartz veins are mainly in the Ordovician and Silurian rocks; but some also occur in the Devonian, and there are impregnations of gold in tufas of Devonian age.
Silurian, head showing the labrum and Lanark.
The Phyllopoda, Ostracoda and Cirripedia (Thyrostraca) are represented in Cambrian or Silurian rocks by forms which seem to have resembled closely those now existing, so that palaeontology can have little light to throw on the mode of origin of these groups.
The central and most picturesque part of the district is formed of great masses of volcanic ashes and tuffs, with intrusions of basalts and granite, all of Ordovician (Lower Silurian) age.
Sedimentary rocks of the same age form a belt to the north, and include Skiddaw (3054 ft.); while to the south a belt of Silurian rocks, thickly covered with boulder clay, forms the finely wooded valleys of Coniston and Windermere.
Mid Wales is built up, for the most part, of Silurian or Ordovician rocks, practically free from igneous intrusions except in the south-west.
The group of heights of South Wales, running on the whole from west to east, marks the outcrops of the Old Red Sandstone and Carboniferous strata which lie within a vast syncline of the Silurian rocks.
Around the western and northern edge of the Old Red Sandstone plain the underlying Silurian rocks (and even the Cambrian and Archaean in places) have been bent up so that their edges form hills of singular abruptness and beauty.
Wenlock Edge, running from south-west to north-east, is an escarpment of Silurian limestone, while the broad upland of Long Mynd, nearly parallel to it on the north, is a mass of Archaean rock.
We may note the pre-Cambrian lavas and tuffs of the Wrekin district in Shropshire and the somewhat later volcanic rocks of Charnwood; the porphyrites, andesites, tuffs and rhyolites of the Borrowdale volcanic centre, erupted in the Ordovician period, and the Silurian granites of the same region.
The western and eastern shores consist of boulder clay, as well as a narrow strip on the southern shore, south of which runs a ridge of crags of Silurian sandstones.
Geologically, Banka resembles the Malay Peninsula, its formations being mainly granite, Silurian and Devonian slate, frequently covered with sandstone, laterite (red ironstone clay) of small fertility, and alluvium.
Regarding now the outcrops of bed-rock, there are exposures of Algonkian (doubtful, and at most a mere patch on Pilot Knob), Archean, Cambrian, Ordovician, Silurian, Devonian, sub-Carboniferous and Carboniferous.
Along the margin of this great deposit, on the Mississippi river below St Louis and along the northern shore of the Missouri near its mouth, is an outcrop of Silurian.
Both this and the Silurian are mere fringes on the great area of CambroOrdovician.
Thus, for example, on woo acres or less, it commonly falls to onetenth of a cubic foot, and upon an upland Silurian area of 940 acres, giving no visible sign of any peculiarity, the discharge fell, on the 21st of September 1893, to one-thirty-fifth of a cubic foot per second per woo acres.
Among the sedimentary rocks we have, for example, in the clay slates of the Silurian formations, rocks no less cracked and fissured than others, but generally quite impermeable by reason of the joints being packed with the very fine clay resulting from the rubbing of slate upon slate in the earth movements to which the cracks are due.
Silurian formation, used' in the case of the Vyrnwy dam, had an ultimate crushing strength of from 700 to 1000 tons per sq.
The order was more abundantly represented in Palaeozoic times by the Heliolitidae from the Upper and Lower Silurian and the Devonian, and by the Thecidae from the Wenlock limestone.
The Dendroidea alone, however, have this extended range, the Graptoloidea becoming extinct at the close of Silurian time.
Both groups make their first appearance together near the end of the Cambrian; but while in the succeeding Ordovician and Silurian the Dendroidea are comparatively rare, the Graptoloidea become the most characteristic and, locally, the most abundant fossils of these systems.
Those plates are perhaps constant throughout sea-urchins and starfish (though it would puzzle any one to detect them in certain Silurian echinoids), and they may be traced in some of the fixed echinoderms; but there is no proof that they represent the radials of a simple crinoid, and there are certainly many cystids in which no such plates existed.
The Silurian genera Eucladia and Euthemon have the rays greatly reduced and merged in the disk, so that the ambulacrals are unseen.
The Silurian, Devonian and Carboniferous systems are represented - the Silurian and Devonian forming the greater part of the hills S.E.
It is only in the outer foldings of the highlands that Palaeozoic fossiliferous deposits are found - Silurian, Devonian, Carboniferous and Permo-Carboniferous.
The great difference in character, however, between the Silurian strata at Pomeroy in county Tyrone and the adjacent metamorphic series makes it highly probable that the latter masses are truly Archean.
Even if the volcanic and intrusive basic rocks prove to be Ordovician (Lower Silurian), which is very doubtful, the metamorphic series of the core is clearly distinct, and appears to be " fundamental " so far as Ireland is concerned.
Their rocks have been variously held to be Archean, Cambrian and Silurian, and their general trend has undoubtedly been determined by post-Silurian earth-movements.
Following on these rocks of unknown but obviously high antiquity, we find fossiliferous Ordovician (Lower Silurian) strata near Killary harbour on the west, graduating upwards into a complete Gotlandian (Upper Silurian) system.
Here the Ludlow strata are followed by a thick series of barren beds (the Dingle Beds), which have been variously claimed as Upper Silurian and Lower Devonian.
No certain representative of the Dingle Beds has been traced elsewhere throughout the south of Ireland, where the Old Red Sandstone succeeds the uptilted Silurian strata with striking unconformity.
The Silurian rocks were indeed greatly folded before the Old Red Sandstone was laid down, the general trend of the folds being from south-west to north-east.
On either flank the Silurian shales, slates and sandstones, which are very rarely fossiliferous, rise with steep dips.
The foothills and lowlands throughout southern Wicklow and almost the whole of Wexford, and the corresponding country of western Wicklow and eastern Kildare, are thus formed of Silurian beds, in which numerous contemporaneous and also intrusive igneous rocks are intercalated, striking like the chain N.E.
All across Ireland, from the Ballyhoura Hills on the Cork border to the southern shore of Belfast Lough, slaty and sandy Silurian beds appear in the axes of the anticlinal folds, surrounded by Old Red Sandstone scarps or Carboniferous Limestone lowlands.
These Silurian areas give rise to hummocky regions, where small hills abound, without much relation to the trend of the axis of elevation.
These occurrences of granite, with that of Leinster, in connexion with the folding of the Silurian strata, make it highly probable that many of the granites of the Dalradian areas, which have a similar trend and which have invaded the schists so intimately as to form with them a composite gneiss, date also from a post-Silurian epoch of earth-movement.
Certain western and northern granites are however older, since granite boulders occur in Silurian conglomerates derived from the Dalradian complex.
A large exposure of this Old Red Sandstone stretches from Enniskillen to the Silurian beds at Pomeroy, and some contemporaneous andesites are included, reminding us of the volcanic activity at the same epoch in Scotland.
The Slieve Bloom Mountains are thus formed of a dome of Old Red Sandstone folded on a core of unconformable Silurian strata; while in several cases the domes are worn through, leaving rings of Old Red Sandstone hills, scarping inwards towards broad exposures of Silurian shales.
I Silurian (and Cambrian?) "Dalradian" Metamorphic Series .Diorite and allied Basic Rocks '?-t =i=' Granite and allied Acid Rocks montory the conglomerates of this period rest with striking unconformity on the Dingle Beds and Upper Silurian series.
Loughs Mask and Corrib are thus bounded on the west by rugged Silurian and Dalradian highlands, and on the east appear as mere water-filled hollows in the great limestone plain.
Archaean granite is thus exposed at Yampol and other places in Russia, and this is followed towards the west by Silurian and Devonian beds in regular succession - the Devonian being of the Old Red Sandstone type characteristic of the British Isles and of Northern Russia.
They are unfossiliferous, and in the absence of undoubted Cambrian, Ordovician and Silurian strata in Africa they may be regarded as of older date than any of these formations.
Some unfossiliferous conglomerates, sandstones and dolomites in South Africa and on the west coast are considered to belong to the Cambrian, Ordovician and Silurian formations, but merely from their occurrence beneath strata yielding Devonian fossils.
In Cape Colony the Silurian age of the Table Mountain Sandstone is based on such evidence.
Since Plumulites appears to be a synonym of Turrilepas (not Turrilepis), the species Turrilepas wrightii (Woodward, 1865), from the Upper Silurian of Dudley, did not long enjoy an isolated eminence as the oldest known cirripede.
Dr Bather justifiably anticipates further discoveries, but if, already in Silurian as in modern times, the members of these families had to pass through nauplius and cypris stages to maturity, there is one " enormous gap " between them and the common ancestor of the crustacean class that will not be easily filled.
The Silurian system, however, is well developed in north-western Tasmania, and is.
They rest unconformably on the Silurian rocks on the King river and to the west are faulted against the schists by a powerful overthrust fault, traversing the Mount Lyell copper field.
The Devonian period, as in Victoria, was marked by a series of granitic intrusions, which altered the older beds on the contact, while the quartz-porphyry dikes, which are intrusive in the Silurian rocks at the Mount Bischoff tin mine, doubtless belong to this period.
Girvanella, found in Cambrian, Ordovician and Silurian rocks, as well as in later deposits, appears to have played a part in the origination of oolitic rock-structure.
These forms, and others like them, go back to the Silurian and Ordovician; while Gyroporella, from the Permian, is another fairly characteristic Siphoneous type.
Storriei, from the Silurian (Wenlock) of South Wales, described by Barber, there is no sharp differentiation of the two kinds of tubes; they are rarely observed to branch, except in the gaps, which in this species are not radially directed.
Pachytheca, a spherical organism, usually about the size of a small pea, found in rocks of Silurian and Devonian age, has been much investigated and discussed, without any decisive light having been thrown on its nature.
They extend back through the Devonian, possibly to the Silurian system, but the systematic summary now to be given is based primarily on the rich materials afforded by the Carboniferous and Permian formations, from which our detailed knowledge of Palaeozoic plants has been chiefly derived.
From the Ordovician and Silurian, however, a certain number of authentic remains of Algae (among many more that are questionable) have been investigated; they are for the most part either verticillate Siphonae, or the large - possibly Laminariaceous - Algae named Nematophycus, with the problematical but perhaps allied Pachytheca.
The evidence for terrestrial Silurian vegetation is still dubious; apart from some obscure North American specimens, the true nature of which is not established, Potonie has described well-characterized Pteridophytes (such as the fern-like Sphenopteridium and Bothrodendron among Lycopods) from supposed Silurian strata in North Germany; the horizon, however, appears to be open to much doubt, and the specimens agree so nearly with some from the Lower Carboniferous as to render their Silurian age difficult of credence.
The high development of the terrestrial flora in Devonian times renders it probable that land-plants existed far back in the Silurian ages, or still earlier.
Abstract Fossil evidence of terrestrial vascular plant life and terrestrial arthropods exists from the Silurian.
Silurian Surveys The HWDT research vessel Silurian is the perfect platform from which to study Hebridean cetaceans.
Mountain summits show roof of Silurian hornfels above intrusive granites of Western Mournes.
This type of sea scorpion is from the Silurian period.
Here, although exposures are very poor, the Silurian rocks comprise shales, sandstones and limestones with a couple of beds of lava.
Look for angular black colored hard slate as these rocks can contain Trilobites from the Silurian shales.
Dendrograptidae, containing fossil (Silurian) genera, such as Dendrograptus and Thamnograptus, of doubtful affinities.