September 22, 1911] 



SCIENCE 



365 



explained only as the result of repeated 

 differentiation. The distribution of some 

 of the groups indicates the existence at this 

 late stage of subsidiary centers of differen- 

 tiation, distinct from the plutonic centers. 

 Thus, trachyte dykes are found especially 

 throughout a tract extending from the 

 southwestern part of Skye through the 

 middle of Argyllshire, while there is an iso- 

 lated area of these dykes about Drynoch, 

 on the opposite side of the Skye mountains. 

 Here we have an evidently alkaline type. 

 On the other hand, there are rocks which, 

 taken by themselves, must be assigned to 

 the calcic division. Augite-andesites, for 

 example, are well known, especially in 

 parts of western Argyllshire, in Arran and 

 the Cumbraes, and in the outlying districts 

 of the north of Ireland, Anglesey and the 

 northeast of England. That these rocks 

 have arisen as products of a subsidiary dif- 

 ferentiation, we have in some cases almost 

 ocular demonstration ; for in Arran and 

 elsewhere augite-andesites are found in re- 

 markably intimate association with com- 

 plementary types, often pitchstones of alka- 

 line composition. 



Even from so brief and imperfect a 

 sketch we may, I think, draw some conclu- 

 sions which have a wider application. This 

 province exemplifies at once the two main 

 tectonic types, and also comprises repre- 

 sentatives of the two great branches of 

 igneous rocks. Those rocks which are re- 

 lated to broad movements of Atlantic type 

 indicate a parent magma of decided, 

 though not strongly marked, alkaline na- 

 ture; while those related to local move- 

 ments of Pacific type clearly come from a 

 calcic magma. There are some facts which 

 suggest that the rocks tend to become more 

 alkaline as we recede from the chief centers 

 of activity, and this suggestion applies 

 to some calcic as well as alkaline groups of 

 rocks. Finally, it appears that the relative 



simplicity of arrangement was disturbed 

 at a late stage by the effects of subsidiary 

 differentiation, the province tending then 

 to break up into districts related to new 

 centers. Operating upon an initial magma 

 not very strongly characterized, this later 

 differentiation has even given rise to aber- 

 rant rock-types which overstep the petro- 

 graphical boundary line between the two 

 branches. 



PETROGENESIS AND SYSTEMATIC PETROGRAPHY 



From such considerations as I have has- 

 tily passed in review, it is evident that a sur- 

 vey of igneous rocks as they actually occur 

 in the field leads to a conception of their 

 mutual relationships very different from 

 that embodied in the current schemes of 

 systematic petrography. It may be of 

 some interest, in conclusion, to expand 

 this remark a little farther, although I am 

 sensible that in so doing I lay mj'-self open 

 to the charge of vain speculation. 



From the petrogenetic point of view, the 

 most fundamental division among igneous 

 rocks is that between the alkaline and cal- 

 cic branches. This result, independently 

 arrived at on petrographical grounds by 

 several authorities, seems to be firmly es- 

 tablished by the broad distribution of the 

 two branches in different regions of the 

 globe. But, if this argument be admitted, 

 it follows that the next step in a natural 

 grouping of igneous rocks should be sug- 

 gested by a comparison of the character- 

 istics of the various provinces into which 

 the great regions divide. Many of these 

 provinces have now been partly studied,, 

 and their special characteristics can often 

 be expressed in concise terms: e. g., among 

 all^aline rocks the relative proportion of 

 potash to soda may be a characteristic com- 

 mon to a whole province. More precisely, 

 by averaging the chemical analyses of the 

 chief rock-types, weighted according to 



