452 WHITMAN CROSS 



decades with ever improving facilities and methods, has added a 

 vast store of knowledge concerning the characters of these 

 objects. Revelations concerning the composition of types long 

 known have often been astonishing. The essence of rock struc- 

 tures has been made clear. The uttermost parts of the earth 

 have been searched, and many new and interesting varieties have 

 been discovered — not all in distant fields, but often close at 

 hand. For new structures and new types, new terms have been 

 proposed, and the nomenclature has thus expanded enormously. 

 In the light of new discoveries, old conceptions have given way 

 to new ideas, and the terms expressing them have yielded to 

 new ones, or, in too many cases, the old nomenclature has been 

 retained with new definitions. But while the last third of the 

 nineteenth century may be termed the microscopical period of 

 petrography, great additions to our knowledge of rocks were also 

 made in this period on all older lines of study, and especially 

 by quantitative chemical analysis and by investigations as to the 

 modes of occurrence and the field relations of different rocks. 



This review, which deals with system, must trace the applica- 

 tion of new or revised principles to classification during this 

 period of rapid addition to knowledge. It is perhaps quite 

 natural that the greater part of the systematic advance was in 

 partially worked out revisions of old schemes — grafting some 

 new idea to the old trunk. It is also natural that the greatest 

 work has been in the field to which the microscope has been 

 particularly applied, so that at times petrography has been 

 treated as though narrowed to the microscopical petrography of 

 igneous rocks. 



While the flood of microscopical rock studies was at its 

 height, it was manifestly impossible for anyone to do more than 

 to present the new information in comprehensive form, without 

 finished attempt to apply it to the systematic arrangement of 

 rocks. It is with this condition in mind that the important 

 works upon microscopical petrography, issued in the decade 

 1870 to 1880, must be judged. Their actual effect upon the 

 systematic science was, however, very great, from the mere fact 



