118 GEOLOGY OF THE HIGH PLATEAUS. 



which are nearly pure calcic and magnesian carbonates ; we have clay- 

 slates, serpentines, chloritic, and mica schists, which have a composition 

 not at all similar to that of erujjtive rocks. But while a large proportion 

 of the metamorphic rocks have no chemical correspondence to the eruptive 

 rocks, there is another large proportion of them in which the constituents 

 correspond almost exactly to those of the eruptives. These are the gneisses, 

 the hornblendic and the augitic schists. The greater part of the true 

 gneissic rocks yield by analysis practically the same results as granite, 

 syenite, rhyolite, and acid trachyte. The hornblendic schists have about 

 the same constituents as the diorites, propylites, and hornblendic trachytes, 

 while the more basic hornblendic (sometimes augitic) schists hold the same 

 relation to diabase, dolerite, and augitic andesite Thus, then, we find that 

 the eruptive masses have their representatives (chemically considered) 

 among certain groups of metamorphic rocks. 



2d. Metamorphic and igneous rocks compared with respect to mineral com- 

 ponents. — Chemical identity or similarity implies no necessary and exact 

 correspondence in mineral constituents, for the minerals which may be 

 formed in a rockmass under varying conditions of temperature and environ- 

 ment cannot be determined solely by the chemical composition of the 

 ' magma. The crystals of the metamorphic rocks are formed according to 

 the commonly accepted theory of metamorphism, at rather low or very 

 moderate temperatures, while the crystals of igneous rocks are in part at 

 least, and perhaps wholly, generated at high temperatures. Hence it is 

 not surprising that metamorphic rocks should contain some crystalline 

 forms which are seldom or never found in the igneous except as alteration 

 products, or should contain some forms in abundance which the latter con- 

 tain very sparingly. There are, however, some minerals wdiich may be 

 formed indifferently at high or low temperatures, and the most important of 

 these are undoubtedly feldspar and hornblende. Those which form with 

 great facihty at low temperatures are certain forms of mica, quartz, chlorite, 

 and the zeolites, and those which seem to be associated with higher tem- 

 peratures are leucite, nephelin, olivin, and less decidedly augite. By a 

 comparison of the two classes of rocks, therefore, we find an agreement in 

 respect to those minerals which are indifferent to variations of conditions; 



