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Show CLASSIFICATION OF THE EEUPTIVE EOCKS. 101 nate minerals, hornblende, mica, magnetite, &c. It also possessed in nearly all varieties that coarse, rough texture from which the term trachyte originated. The validity of this distinction has been well established by later investigators, and in Germany and America it is universally accepted. To the more acidic division Richthofen gave the name Rliyolite, and preserved the name trachyte for the remainder of the older acidic semi-class. Thus far we are able to subdivide the volcanic rocks into four parts or groups instead of two, as was usually done in the time of Durocher. The older acidic semi-class may be resolved into two groups, the Bhyolites and Trachytes, while the basic semi-class may be resolved into two, the Ande-sites and Basalts. Now, these four groups represent in a very decided manner a progression in the chemical constitution, and also correlative progressions in mineral constitution, in specific gravity, &c. The rhyolites are at the acidic end of the scale of progression and the basalts at the basic end. The trachytes may be called sub-acid rocks and the andesites sub-basic rocks, thus : Acid rocksâ€"RHYOLITES. Sub-acid rocksâ€"TRACHYTES. Sub-basic rocksâ€"ANDESITES. Basic rocksâ€"BASALTS. We shall find further on that this progression is not perfectly rigorous and exact, but presents certain apparent anomalies; that some rocks, for instance, which ought to be and are rationally called andesite are more acid than some rocks which are with equal reason called trachytes. Yet, on the whole, the progression is strongly pronounced and unmistakable, and the seeming anomalies do not invalidate the general law. If we considered chemical constitution alone, however, we should be unable to determine the relative position of any rock in the lithological scale without a chemical analysis. The patent evidence of its position and character is found in the minerals it contains. These, it has already been asserted, are determined by the chemical constitution, and in return indicate that constitution. Each group of rocks has its characteristic group of minerals, of which some may be regarded as essential to the diagnosis of the rock, while others are merely " accessory," being generally present, but |
