When the crystals of the earlier generation are numerous and needle-like in form, as is very often the case, they arrange themselves “end on” during the rock flow, so that when consolidation has occurred, the rock has a kind of puckered lamination which is the characteristic of the _fluxion_ or _flow_ texture. This texture has sometimes been confused with the lamination of the sedimentary rocks, so that wrong conclusions have been reached regarding origin. At other times the same needle-like crystals within the lava have grouped themselves radially to form rounded nodules called spherulites. Such nodules give to the rock a _spherulitic_ texture, which is nowhere better displayed than in the beautiful glassy lavas of Obsidian Cliff in the Yellowstone National Park.
FIG. 17.—Characteristic textures of igneous rocks. _a_, granitic texture characteristic of the deep-seated intrusive rocks; _b_, porphyritic texture characteristic of the extrusive and of the near-surface intrusive rocks; _c_, glassy texture of an extrusive rock.]
Those intrusive rocks which consolidate deep below the earth’s surface, part with their heat but slowly, and so the process of crystallization is continued without interruption. Starting from many centers, the crystals continue to grow until they mutually intersect in an interlocking complex known as the _granitic_ texture (Fig. 17 _a_).
=Classification of rocks.=—In tabular form rocks may thus be classified as follows:—
{ _Intrusive._ Granitic or porphyritic { texture. _Igneous._ Massive and { _Extrusive._ Glassy or porphyritic with sharply angular { texture; often also with vesicular, grains. { scoriaceous, pumiceous, fluxion, { or spherulitic textures.
{ _Subaërial._ Sands and loess. { _Subaqueous._ (See below.) _Sedimentary._ Laminate { _Glacial._ Coarse, unstratified and with rounded { deposits with faceted pebbles. Till grains. { and tillite. { _Fluvio-glacial._ Stratified sands { and gravels with “worked over” { glacial characters.
_Metamorphic._ Schistose { _Metamorphic proper._ Due to below and with grains either { surface changes. angular or rounded. { _Residual._ Disintegrated at or near { surface.
=Subdivisions of the sedimentary rocks.=—While the eolian sediments are all the product of a purely mechanical process of lifting, transportation, and deposition of rock particles, this is not always the case with the subaqueous sediments, since water has the power of dissolving mineral substance, as it has also of furnishing a home for animal and vegetable life. Deposited materials which have been in solution in water are described as _chemical_ deposits, and those which have played a part in the life process as _organic_ deposits. The organic deposits from vegetable sources are peat and the coals, while limestones and marls are the chief depositories of the remains of the animal life of the water. The tabular classification of the sediments is as follows:—