OCR Text |
Show COMPARISON OF ERUPTIVE WITH MET AMORPHIC ROCKS. 119 and disagreement onfy in those minerals which are decidedly dependent upon variations of condition. The metamorphics abound in low temperature minerals, the eruptives in high temperature minerals. Both classes contain abundant feldspar, mica, and hornblende, which seem to be but little affected by temperature, so far as concerns the facility with which they are formed. 3d. Metamorphic and igneous rocks compared with respect to mechanical texture.â€"In the modes of aggregation of the rock-forming materials, the two classes of rocks differ radically. Nor could we anticipate any agreement here. The metamorphics have not been melted down, but retain with greater or less distinctness their original foliation. The changes have been purely molecular. Where the metamorphism is complete the rock is ordinarily made up of purely crystalline matter, each crystal being a definite mineral species, with definite optical and crystallographic properties peculiar to its kind, the whole interlocked into a mosaic of great beauty, which is revealed to the eye by a polished surface, or still more clearly by a thin section under the microscope. But the volcanic rocks have a totally different texture, of which the distinguishing characteristic is the presence of a non-crystalline or amorphous base in which crystals are disseminated. Sometimes the crystals are wholly absent, and the amorphous base-constitutes the entire rock, as in pitchstone and obsidian. The distinction, then, between the texture of a thoroughly metamorphic rock and an extravasated mass is that the former is wholly crystalline, while the latter is either partially or wholly amorphous. And yet we have rocks which present every shade of transition between the two textures. The gneisses, for instance, lose their foliation and become indistinguishable from granites. The granites present varieties which have larger and more perfect crystals imbedded in a maze of smaller ones. We may select a series in which the mosaic of surrounding crystals becomes finer and finer and the* inclosed crystals more perfect and contrasted, and such a group is called porphy-ritic granite or granite porphyry. Following this chain of varieties, the crystalline base gradually passes into one in which the utmost power of the microscope fails to detect any individualized crystals, but merely indicates by indirection that the base has been in some way influenced by the crys- |