48 THE CANADIAN NATURALIST. [March 



examination of rocks have very much contributed towards render- 

 ing such attempts successful. In the present paper it is proposed 

 to give a systematic view of the various classes and species of 

 crystalline rocks, in arranging which it is intended that their 

 chemical composition shall have greater prominence and weight 

 than has been usual heretofore. 



However much it may seem desirable in this department of 

 science, where all the systems of classification have been con- 

 fessedly imperfect, to invent a system independent altogether of 

 the ideas, more or less well founded, which prevail as to their 

 origin and age, and in which their physical and chemical charac. 

 ters should only have consideration, it must not, on the other 

 hand, be forgotten that what is still more desirable in such a 

 system is that it should re-arrange our knowledge of the subject in 

 a clearer form, render it more easy of comprehension to the 

 student, and be so dovetailed into the past of the science as to be 

 useful for its advancement in the future. On this account it 

 becomes impossible to neglect even the theoretical views of our 

 forerunners in this science of petrology, far less their arduous 

 and often underrated geognostic labours. It also becomes re- 

 quisite to give a proper value to all the considerations which may 

 have influenced their views, and to build upon the foundation 

 which they have left us, the results of the observations and 

 research of the investigators of our own day. 



Considerations as to the manner of formation, texture, chemical 

 and mineralogical composition, age and localities of rocks, have 

 all, more or less, influenced geologists in naming and classifying 

 them. The well-known distinction between eruptive and sedi- 

 mentary rocks will occur to every reader as an instance of classi- 

 fication according to origin. Hunt's division of crystalline rocks 

 into indigenous and exotic, and Scheerer's distinction of plutonites 

 and vulcanites are both founded upon their real or supposed 

 manner of formation. Lava and Rhyolite are examples of special 

 rocks similarly named. Then, with regard to texture, probably 

 no other character possessed by rocks has given rise to a greater 

 number of generic terms. Schist, slate, porphyry, trachyte, 

 amygdaloid, conglomerate, and breccia, are examples of this, but 

 of special names founded on texture only a few can be instanced, 

 such as granite and aphanite. The influence of chemical compo- 

 sition on lithological nomenclature is not, as yet, very marked, 

 for it is only recently that the analysis of rocks has had much 



