CLASSIFICATION OF THE ERUPTIVE ROCKS. 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 Bhyolite, and pre- 

 served 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 pro- 

 gressions 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 rocks— RHYOLITES. 



Sub-acid rocks— TRACHYTES. 



Sub-basic rocks— ANDESITES. 



Basic rocks— 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 min- 

 erals, of which some may be regarded as essential to the diagnosis of the 

 rock, while others are merely " accessory," being generally present, but 



