A complex of igneous rocks of different ages covers immense areas in the central Transvaal.
Tremolite-schist and anthophyllite-schist are in nearly all cases the representatives of the ultra-basic igneous rocks such as peridotite in regions of high metamorphism.
It is certain, however, that most obsidians are very acid or rhyolitic. The dark, semiopaque glassy forms of the basic igneous rocks are known as tachylytes.
Amongst the most interesting features of the Bakony-wald are the volcanic and the igneous rocks.
This consists largely of rocks of igneous origin, of which the amygdaloidal diabase of Klipriversberg forms the type.
It has also been observed as a product of contact-metamorphism in carbonaceous clay-slates near their contact with granite, and where igneous rocks have been intruded into beds of coal; in these cases the mineral has clearly been derived from organic matter.
The most striking feature in the geology of Russia is its remarkable freedom from disturbances, either in the form of mountain folding or of igneous intrusions.
Glasser, the basic igneous rocks which are associated with the mineral deposits of New Caledonia were intrusive in Cainozoic times, at the severing of the connexion between New Caledonia and New Zealand.
In many deposits of iron ores found in connexion with igneous or metamorphic rocks small quantities of phosphate occur.
Staffordshire, the diorites of Warwickshire, the phonolite of the Wolf Rock (to which he first directed attention), the pitchstones of Arran and the altered igneous rocks near the Land's End were investigated and described by him during the years1869-1879in the Quarterly Journal of the Geological Society and in the Geological Magazine.
Lava streams and other signs of volcanic action abound, but there has been no igneous activity since the Spaniards took possession.
The results of his observations were embodied in his Treatise on Primary Geology (1834), a work of considerable merit in regard to the older crystalline and igneous rocks and the subject of mineral veins.
The foundation of the island is formed of metamorphic and igneous rocks, which appear in the Sierra Maestra and are exposed in other parts of the island wherever the comparatively thin covering of later beds has been worn away.
It has been variously attributed to metamorphism, consequent upon igneous intrusion, earth movements and other kinds of geothermic action, greater or less loss of volatile constituents during the period of coaly transformation, conditioned by differences of permeability in the enclosing rocks, which is greater for sandstones than for argillaceous strata, and other causes; but none of these appears to be applicable over more than limited areas.
The first mode of occurrence is of little significance practically, for the crystalline rocks generally contain too little phosphate to be valuable, though occasionally an igneous rock may contain enough apatite to form an inferior fertilizing agent, e.g.
==Geology== The rocks of the Appalachian belt fall naturally into two divisions; ancient (pre-Cambrian) crystallines, including marbles, schists, gneisses, granites and other massive igneous rocks, and a great succession of Paleozoic sediments.
Igneous intrusions consist only of unimportant dikes of trap. The most striking and uniformly characteristic geologic feature of the mountains is their internal structure, consisting of innumerable parallel, long and narrow folds, always closely appressed in the eastern part of any crosssection (Piedmont Plateau to Great Valley), less so along a central zone (Great Valley and Valley Ridges), and increasingly open on the west (Allegheny and Cumberland Plateaus).
Grits, igneous rocks, ashbeds and limestones may and often do show cleavage.
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.
Poas (8895), the scene of a violent eruption in 1834, begins a fresh series of igneous peaks, some with flooded craters, some with a constant escape of smoke and vapour.
The direction of the great volcanic cones, which rise in an irregular line above it, is not identical with the main axis of the Sierra itself, except near the Mexican frontier, but has a more southerly trend, especially towards Salvador; here the base of many of the igneous peaks rests among the southern foothills of the range.
Most of them are the metamorphic products of igneous rocks, among which extrusive rocks, many of them pyroelastic, predominate.
The system is without any notable amount of igneous rock.
The folding, moreover, is less intense; but in the Dolomites of Tirol there are great outbursts of igneous rock, and faulting has occurred on an extensive scale.
Those derived from sedimentary and those derived from igneous rocks, or, as they have been called, the "paraschists" and the "orthoschists."
The chlorite-schists are often of igneous derivation, such as ash-beds or fine lavas which have been metamorphosed.
Much discussion had centred about fire or the "igneous principle."
On the one hand, it had been held that when a substance was burned or calcined, it combined with an " air "; on the other hand, the operation was supposed to be attended by the destruc tion or loss of the igneous principle.
Practically all the remaining area in these islands is occupied by metamorphic schists and gneisses which occur in great variety and with which are associated numerous dikes and masses of intrusive igneous rock.
The igneous formation of which the greater part of the Roman Campagna is, in its superior portion, composed, contains three strata known under the common name of tufa, - the " stony," " granular," and " sandy " tufa, - the last being commonly known as pozzolana.
The close of the period was marked by very profound changes which may be classed under three general headings: (1) the emergence of great areas which had been submerged until the closing stages of the period; (2) the beginning of the development of most of the great mountains of the west; (3) the inauguration of a protracted period of igneous activity, stimulated, no doubt, by the crustal and deeper-seated movements of the time.
The sites of deposition varied as the period progressed, for the warping and faulting of the surface, the igneous extrusions, and the deposition of sediments obliterated old basins and brought new ones into existence.
He seems to have regarded the soul as composed of igneous matter, and so approximates the orthodox Pythagorean doctrine of the central fire, or Hestia, to the more detailed theories of Heraclitus.
They are generally of lenticular form, and usually occur in or near the contact of eruptive rocks with schists or slates; the presence of the igneous rock being probably connected genetically with their origin.
Pectolite is a secondary mineral occurring as white masses with a radially fibrous structure in the veins and cavities of basic igneous rocks.
The igneous rocks occur at several geological horizons, but the great volcanic eruptions did not begin until the Tertiary period.
On the Columbia plateau the soil is principally volcanic ash and decomposed lava; it is almost wholly volcanic ash in the more arid sections, but elsewhere more decomposed lava or other igneous rocks, and some vegetable loam is mixed with the ash.
An interesting and orderly petrological sequence of Tertiary igneous rocks has been determined.
Locally both the sedimentary and igneous parts of the group have been highly metamorphosed; but as a rule the alteration of the sedimentary portions has not gone so far that stratigraphic methods are inapplicable to them, though in some places detailed study is necessary to make out their structure.
The commoner sorts of rock in the several Huronian systems are quartzite and slate (ranging from shale to schist); bi~t limestone is not wanting, and igneous rocks, both intrusive and extrtisive, some metamorphic and some not, abound.
The sedimentary rocks are affected by many dikes and sheets of igneous rock, some of the latter being extrusive and some intrusive.
Igneous rocks, whether lava flows or pyroclastic ejections, are less important in the Quaternary than in the Tertiary, though volcanic activity is known to have continued into the Quaternary.
The rolled crystals of sapphire occur, with garnet and other minerals, in glacial deposits, and have probably been derived from dykes of igneous rocks, like andesite and lamprophyre.
As it is, it has very fertile patches of lowland, such as the plains of Esdraelon and Jaffa; and the high levels, largely composed of disintegrated igneous rock, west of Jordan, over which the seawind carries the rains, offer excellent corn-land.
They occur (a) in crystalline igneous and metamorphic rocks as an original constituent, (b) in veins associated with igneous rocks, and (c) in sedimentary rocks either as organic fragments or in secondary concretionary forms.
"water," 12,139 ft.), so named in 1541 because it destroyed the forme(capital of Guatemala with a deluge of water from its flooded crater; and Pacaya (8390), a group of igneous peaks which were in eruption in 1870.
Igneous and metamorphic rocks.
Those rocks which contain andalusite and staurolite are sometimes found in such associations as show that they are due to contact action by intrusive igneous masses.
Crystals are found as a rarity in the amygdaloidal cavities of igneous rocks.
Many of the oceanic islets are composed of coral limestone, which in this way becomes phosphatized; others are igneous, consisting of trachyte or basalt, and these rocks are also phosphatized on their surfaces but are not so valuable, inasmuch as the presence of iron or alumina in any quantity renders them unsuited for the preppration of artificial manures.
While igneous and metamorphic crystalline rocks form the bulk of the Adirondack area, it is surrounded by a ring of ancient Palaeozoic sediments in which these peripheral lowlands have been developed.
Its superstructure consists of igneous rocks of all descriptions with which the original valleys between its marginal ranges have been filled by volcanic action.
In North America, the principal region of volcanic activity lay in the west; great thicknesses of igneous rocks occur in the Lower Carboniferous rocks of British Columbia, and from the middle of the period until near its close volcanoes were active from Alaska to California.
The whole group is abundantly watered, and the igneous soil is marvellously fertile.
Thus he carried on the narrative of orderly development from the point at which it was left by Kant and Laplace - explaining by reference to the ascertained laws of physics and chemistry the configuration of the earth, its mountains and seas, its igneous and its stratified rocks, just as the astronomers had explained by those same laws the evolution of the sun and planets from diffused gaseous matter of high temperature.
The value of the product (including a small output of igneous rocks) was in 1903, $ 2, 35 1, 02 7; 1904, $ 2, 554,74 8; 1905, $ 2, 2 5 1, 3 1 9; 1906, $3,3 2 7,4 16; 1907, $ 2, 3 28, 777; 1908, $2,027,463.
Another group of phosphatic deposits connected with igneous rocks comprises the apatite veins of south Norway, Ottawa and other districts in Canada.
They are soft and lustrous, with a peculiarly smooth feel, and though often confounded with mica-schists may be distinguished by their richness in magnesia; many of them contain tremolite or actinolite; others have residual grains of olivine or augite; and here also every gradation can be found between the unmodified igneous types and the perfectly metamorphic schists.
The lower part of the Keweenawan system consists of a great succession of lava flows, of prodigious thickness.- This portion of the system is overlain by thick beds of sedimentary rock, mostly conglomerate and sandstone, derived from the igneous rocks beneath.
The typical obsidians exhibit the chemical peculiarities of the acid igneous rocks (viz.