In the centre of the folds fossiliferous beds with crinoids have been found, and the black slates at the top of the series contain Myophoria and other fossils, indicating that the rocks are of Triassic age.
Slates and fine-grained sandstones appear here freely through the glacial drift.
A structure akin to cleavage, often exemplified by slates especially when they have been somewhat contorted or gnarled, is the Ausweichungsclivage of Albert Heim.
The faces of slates have usually a slightly silky lustre due to the abundance of minute scales of mica all lying parallel and reflecting light simultaneously from their pearly basal planes.
In microscopic section the best slates show much colourless mica in small, thin, irregular scales.
Minute rods or needles of rutile are also common in slates, and well-formed cubes of pyrites are often visible on the splitting faces.
The brownish colour of some slates is due to limonite and haematite, but magnetite occurs in the darker coloured varieties.
By advancing crystallization and increased size of their components, slates pass gradually into phyllites, which consist also of quartz, muscovite and chlorite.
In the neighbourhood of intrusive granites and similar plutonic igneous rocks, slates undergo "contact alteration," and great changes ensue in their appearance, structure and mineral composition.
Farther away from the granite the slates are not so much altered, but generally show small rounded or ovoid spots, which may be darker or lighter in colour than the matrix.
In these spotted slates andalusite, chiastolite, garnet and cordierite often occur; chiastolite is especially characteristic; cordierite occurs only where the alteration is intense.
In other cases, especially near mineral veins, slates are filled with black needles of tourmaline or are bleached to pale grey and white colours, or are silicified and impregnated with mineral ores.
Frequently in districts where slates are much crumpled they are traversed by numerous quartz veins, which have a thickness varying from several inches up to many feet, and may occasionally be auriferous.
F.) Slates are widely used for roofing houses and buildings of every description, and for such purposes they are unequalled, the better sorts possessing all the qualities necessary for protection against wind, rain and storm.
" Lean-to," " shed," or " pent " roofs are practically developments of the flat roof, one end of the joists (which are now called " rafters ") being tipped up to form a decided slope, which enables slates, tiles, corrugated iron and other materials to be employed which cannot be used upon a " flat " roof.
The Cambrian is represented by blue clays, ungulite sandstones and bituminous slates in Esthonia and St Petersburg.
Schists in the common acceptance of that term are really highly crystalline rocks; fissile slates, shales or sandstones, in which the original sedimentary structures are little modified by recrystallization, are not included in this group by English petrologists, though the French schistes and the German Schiefer are used to designate also rocks of these types.
The phyllites (q.v.) form a middle term between this group and the slates; they consist usually of quartz, white mica and chlorite, and have much of the foliation and schistosity of the mica-schists.
Abich (Sur la structure et la geologic du Daghestan, 1862), the successive folds of Jurassic limestones and slates, all nearly parallel to the Caucasus, which form lofty, narrow plateaus.
The southern part of Mainland, from Laxfirth Voe to Fitful Head a series of dark schists and slates, is found with subordinate limestones.
The principal imports are coal, timber and slates, and the principal export stone of the Transition limestone or Devonshire marble.
- Restoration of Triarthrus Becki, Green, as determined by Beecher from specimens obtained from the Utica Slates (Ordovician), New York.
Walchi is abundant in the Oolitic lithographic slates of Bavaria.
They show a great variety of type made up of slates, quartzites, occasional conglomerates, schists with large masses of intrusive granites and gneiss.
The lower group (Hospital Hill slates) consists of quartzites and shales, resting on the eroded surface of the older granites and schists, and estimated to be from 10,000 to 12,000 ft.
The Black Reef Series is composed of quartzites, sandstone, slates and conglomerate.
In the Marico district the shales become highly ferruginous and resemble the Hospital Hill slates of the Witwatersrand series.
Most of the island is occupied by the band of the old rocks, which include mica, glaucophane and sericite-schists and slates; there are small intrusions of granite, and numerous dikes and masses of basic eruptive rocks.
The slates are interbedded with limestones containing fossil brachiopods, which have led to their determination as Silurian or Devonian; but L.
BALA SERIES, in geology, a series of dark slates and sandstones with beds of limestone which occurs in the neighbourhood of Bala, Merionethshire, North Wales.
Of slates from the Bala limestone, below this are more slates and volcanic rocks.
Slates properly so-called do not, except on rare occasions, split along the bedding, but along planes of cleavage, which intersect the bedding usually at high angles.
The original material was a fine clay, sometimes with more or less of sand or ashy ingredients, occasionally with some lime; and the bedding may be indicated by alternating bands of different lithological character, crossing the cleavage faces of the slates, and often interrupting the cleavage, or rendering it imperfect.
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.
The action of pressure is shown also by the fossils which sometimes occur in slates; they have been drawn out and distorted in such a way as to prove that the rock has undergone deformation and has behaved like a plastic mass.
If the bedding be traced, either in the slates or in the other rocks which accompany them, flexures will be frequently observed (the folding often being of an isoclinal type), while reversed faulting, or thrusting, is usually also conspicuous.
This structure is by no means confined to slates, though always best exemplified in them, owing probably to the finegrained, argillaceous materials of which they consist.
The cleavage of slates must be distinguished from cleavage of minerals, the latter being due to different degrees of cohesion along definite crystallographic planes.
This method of reasoning, however, does not carry us far, as the minerals of slates vary considerably in form.
Pressure will also tend to produce an expansion of the rock mass in a direction (usually nearly vertical) at right angles to the compression, for such rocks as slates are distinctly plastic in great masses.
Slates consist largely of thin plates of mica arranged parallel to the cleavage faces.
Micas and other platy minerals (such as chlorite), which naturally grow most rapidly on their edges, would show this tendency best, and such minerals usually form a large part of the best slates; but even Sketch (by Du Noyer) of a block of variegated slate from Devil's Glen, Co.