498 WHITMAN CROSS 



graphic system, has been proposed. In the existing arrange- 

 ments the confusion of correlating various cross-classifications 

 into one whole is quite evident. 



4. Metamorphic rocks, including all such in which the derived 

 characters are more prominent than the original ones, defy sys- 

 tematic treatment at the present time. Since they have been 

 formed from all kinds of original rocks, by many different pro- 

 cesses, and at many sites in the earth's mass, there are many 

 standpoints from which they may be considered, and their classi- 

 fication is a complex problem. Among the facts most difficult 

 to recognize in system are the close resemblance or identity of 

 metamorphic products from originally different rocks, and the 

 similar correspondence between certain secondary and primary 

 rocks. The proposition of Walther to classify all metamorphics 

 with the masses from which they were derived is thus imprac- 

 ticable at the present time, even if it be thought desirable. In 

 relation to this class of rocks systematic petrography is in the 

 condition that its arrangements are tentative, awaiting new 

 knowledge concerning the genesis and essential characters of 

 the objects. 



The review of the development of systematic petrography 

 given in the preceding pages has been mainly a discussion of 

 comparatively comprehensive arrangements or systems which 

 have been proposed. It is, of course, true that these systems 

 are but correlations of ideas from many sources, and a complete 

 history of the subject wou|d give to important discoveries of 

 fact and to critical or creative suggestions their due weight in 

 influencing the development of systems. But such influence is 

 difficult to trace, and to have attempted such a history would 

 have involved the expenditure of much more time than the 

 writer could devote to the subject. For this reason a large 

 number of important essays, bearing upon certain features of 

 classification or devoted to discussion of principles, have been 

 left unnoticed because they were not accompanied by general 



