88 THE CANADIAN NATURALIST, [Vol. ix. 



1. The necessities of the science of Geology constitute the most 

 prominent motive for distinguishing kinds of rocks ; and they 

 should determine to a large extent upon what characters dis- 

 tinctions should be based. 



2. In determining the rocks to be grouped as one in Idnd 

 under a common name, near identity in the chemical and mineral 

 composition of the chief constitutents is the main point to be 

 considered ; not near identity in their crystalline forms, for iso- 

 morphism presupposes diversity of composition. 



3. Distinction of hind should be based on difference in chem- 

 ical and mineral constitution as regards the chief constituents. 

 When such difference exists, rocks are different in kind^ and 

 need, for the purposes of geology, distinct names. If it does 

 not exist, the distinction is only that of variety ; unless (as in 

 the case of trachyte and felsyte), the very wide extension of the 

 rock under persistent characters makes a distinction of name 

 important to geology. 



4. It follows from the preceding, that differences in texture : 

 as coarse, or fine, or aphanitic; porphyritic, or non-porphyritic ; 

 Btoney throughout, or having unindividualized portions among 

 the stoney grains, and differences in microscopic inclusions ; are 

 no basis for distinctions of kind among rocks, but only of 

 variety ; and i\ifit porphyritic structure is of hardly more conse- 

 quence than coarse or fine granular. 



5. No marked change in the constituents of the earth's erupted 

 material occurred after the close of the Cretaceous period, or just 

 before the commencement of the Tertiary era ; and hence, no 

 ground exists for the distinction of "older" and "younger" 

 among eruptive rocks. The " younger " eruptive rocks are essen- 

 tially like the "older" in chemical composition and their chief 

 mineral constituents ; and they differ when at all only in texture 

 and some other points of as little importance — qualities that dis- 

 tinguish merely varieties, and which have proceeded from greater 

 prevalence in these later times of subaerial eruptions. 



6. Since " plagioclase " is not the name of a mineral species, — 

 several minerals, of widely different compositions being embraced 

 tinder it — it is a confounding of differences and resemblances to 

 speak of it as a constituent of a rock. And since it now includes, 

 through the defining of the feldspar microcline, a large part of 

 potash feldspar, which had been supposed to be orthoclase, it 



^has become almost synonymous with the term feldspar. The 



