1883.] 415 [Wadsworth. 



alogyas a basis of classification cannot be denied. But its exclu- 

 sive application has caused the combination into certain groups, of 

 such rocks as from a geological point of view are widely sepa- 

 rated, while it has given rise to distinctions in cases where the 

 results of geological observation would demand close connection. 

 ... It can easily be demonstrated that when exclusively applied, 

 it [chemical composition] leads to a systematical arrangement of 

 rocks, which is in even greater contradiction with the natural mode 

 of occurrence than when the same is based upon mineral composi- 

 tion alone. . . . The natural differs from the artificial system in 

 this, that it starts from the application not of one only but of va- 

 rious principles, compares and weighs the results obtained by each 

 of them, and accepts them as final only when perfectly harmoniz- 

 ing among each other. . . . The natural system of rocks should, 

 therefore be based, not only upon the entire range of their petro- 

 graphical characters, such as mineral composition, chemical com- 

 position, texture, and specific gravity, but also upon their mode of 

 origin and geological occurrence. . . . Their recurrence in the 

 most widely separated countries, with similar external character, 

 identical chemical composition, and in analogous relative order of 

 succession, is another distinguishing feature of eruptive rocks. 

 For these reasons, as well as [others], . . . they appear to owe 

 their present positions to the action of general planetary processes, 

 and to reveal by their own nature that of the mineral matter par- 

 ticipating in the original composition of the globe. . . . Bearing 

 in their own character and system the imprint of their origin, the 

 eruptive rocks will, by their nature itself, allow well-founded con- 

 jectures as to the interior structure and composition of the earth." 

 (Mem. Cal. Acad. Sci., 1868, i, part it, pp. 5-9) These principles, 

 so far as they go, are held by me, and constitute in part the basis 

 of my classification ; and in Richthofen's case they constituted 

 " an extraordinary step in scientific progress," according to Zir- 

 kel. Having been informed that Richthofen's system was fully 

 sustained by the 40th Parallel Exploration, it was naturally sup- 

 posed that his principles would be followed and a natural system 

 founded ; instead of this I found, as I conceive, a mongrel system 

 — a new garment patched with very old and tender cloth. This 

 system appeared to me to be entirely artificial, and not even 

 then to have any fixed principles. 



