264 THE CANADIAN NATURALIST. [Yol. Vl. 



It will he observed from this table that a certain degree of 

 consistency is observed by the essential minerals in entering into 

 the constitution of original rocks. Such acid minerals as quartz 

 and orthocl.ise never occur in the basic rocks ; nor, on the other 

 hand, do we find augite or labradorite entering into the composi- 

 tion of siliceousgranites or trachytes. Towards the basic extreme 

 of chemical composition in rocks, the siliceous minerals diminish 

 or disappear, and, towards the acid extreme, basic minerals act in 

 the same way. This behaviour alone is sufficient to bhew that the 

 mineralogic.il constitution of a rock is not the result of accident, 

 but mainly the consequence of the chemical nature of the plastic 

 m igna from which it resulted, an inference which is borne out 

 by the varying composition of the minerals themselves. 



It will be seen that at the heads of the columns the minerals 

 have been arranged according to the classification already given. 

 Now it would appear, with regard to the members of each of 

 the classes which we have dii^tinguished, that not only do they 

 resemble each other in chemical composition but they seem to 

 replace each other when they enter into the composition of origi- 

 nal rocks. That is to say, the increase, in quantity, of one of them 

 in a rock is generally accompanied by a decrease on the part of 

 another member of the class, and generally of that miCmber which 

 most closely approaches the first in chemical composition. This 

 appears to be well borne out by the table, and numerous examples 

 of such substitutions might be cited. Thus hornblende replaces 

 mica in granitite forming syenite ; oligcclase replaces orthoclase in 

 the passage from syenite to diorite ; and diallage replaces pyrox- 

 ene in that species of greenstone called gabbro. There are thus 

 formed gradual transitions from one rock species to another in 

 mineralogical constitution as well as chemical composition. In 

 the subjoined table (III) the nature and manner of these tran- 

 sitions arc exhibited. It will be seen that the distinctions 

 already made as to the orders and families of rocks are kept 

 steadily in view while at the same time an attempt is made to give 

 a systematic arrangement of the diflferent species of original rocks 

 and their mutual relations. 



