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.
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.
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.
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.
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 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.
The chlorite-schists are often of igneous derivation, such as ash-beds or fine lavas which have been metamorphosed.
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.
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.
Most of them are the metamorphic products of igneous rocks, among which extrusive rocks, many of them pyroelastic, predominate.
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.
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.
Those derived from sedimentary and those derived from igneous rocks, or, as they have been called, the "paraschists" and the "orthoschists."
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.