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Show 118 GEOLOGY OF THE HIGH PLATEAUS. h which are nearty 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 eruptive 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 tvith respect to mineral components.â€"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 environment 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 contain very sparingly. There are, however, some minerals ,wrhich 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 facility at low temperatures are certain forms of mica, quartz, chlorite, and the zeolites, and those which seem to be associated with higher temperatures 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 |
