GOMPAWSON OF ERUPTIVE WITH METAIMORPHIC ROCKS. 119 



and disagreement only in those minerals which are decidedly dependent 

 upon variations of condition. The metamorphics abound in low tempera- 

 ture 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. — In the modes of aggregation of the rock-forming materials, the 

 two classes of rocks differ radically. Nor could we anticipate any agree- 

 ment 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 ordi- 

 narily made up of purely crystalline matter, each crystal being a definite 

 mineral species, with definite optical and crystallographic properties pecu- 

 liar 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 differ- 

 ent 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-constir 

 tutes the entire tock, 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 par- 

 tially or wholly amoi-phous. 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- 



