M2 



SCIENCE. 



[N. S. Vol. XVII. No. 426 



structure was thought to be a peculiar type 

 of hexagonal crystallization. During the 

 early years of the last century progress, it is 

 true, was comparatively slow, but with the 

 introduction of the microscope the science 

 received a tremendous impetus and a great 

 number of enthusiastic workers were attracted 

 to it, so that during the past thirty years or 

 more an immense store of facts has been col- 

 lected. The system of petrographical classi- 

 fication, however, which has been gradually 

 elaborated, while never wholly satisfactory, is 

 now proving inadequate and unwieldy. It 

 fails, moreover, to express the chemical re- 

 lationship of rocks. Every year a host of 

 new names, having in themselves nothing to 

 indicate the character of the rocks which they 

 designate, are being introduced and applied 

 to new types or varieties, and the confusion 

 promises to increase with the advance of our 

 petrographical knowledge. 



An able historical summary of the develop- 

 ment of petrographical classification is given 

 by Cross in the introduction to the volume 

 under consideration, and a new system of 

 classification for the igneous rocks is then 

 presented, based on thoroughly scientific prin- 

 ciples and capable of indefinite expansion so 

 as to meet all requirements of the science as 

 it develops. There is no attempt made in . 

 the new system to remodel any existing sys- 

 tem of classification so as to meet present 

 needs. As the authors state, it would be im- 

 possible to do this satisfactorily. An entirely 

 new classification is presented with an en- 

 tirely new nomenclature — a nomenclature, 

 however, based on mnemonic principles so 

 that it can be grasped and remembered with 

 comparatively little effort. 



Before passing to the examination of this 

 new classification, however, it must be pointed 

 out that the authors really present two systems 

 of classification for the use of geologists — a 

 simple or general classification for use in the 

 field, based altogether on the megascopic char- 

 acters of rocks — and a second much more 

 elaborate and detailed classification which is 

 to be employed after a more complete study of 

 the rock has been made. These two systems 

 are in agreement with each other, the second 



forming in a way an extension or' elaboration 

 of the first. The classification for field pur- 

 poses retains the common names now in gen- 

 eral use, granite, syenite, diorite, basalt, mela- 

 phyre, etc., although in some cases giving the 

 terms a rather more comprehensive meaning 

 than they have at present. Thus syenite is 

 made to include all coarse-grained igneous 

 rocks, rich in feldspar, the feldspar, however, 

 being either orthoclase or plagioclase. The 

 term thus embraces, in addition to the normal 

 syenites, the anorthosites, as well as the more 

 feldspathic monozonites, diorites and gabbros 

 of the present classification. However, it 

 may be said that, so far as the field geologists 

 are concerned, the general classification pro- 

 posed will not diflier from that at present in 

 use to such an extent as to cause any incon- 

 venience in applying it. For them, in fact, 

 petrographie classification is made distinctly 

 easier. 



Themol-e detailed classification is based on 

 the chemical composition of the rocks, all 

 rocks of a like chemical composition being 

 grouped together. The rock is thus classified 

 according to the composition of the magma 

 from which it solidified. The classification 

 is, furthermore, quantitative, and is thus ad- 

 mirably adapted for purposes of comparison 

 and for studies in rock diiferentiation, which 

 are playing so important a part in modern 

 petrographical work. A chemical analysis or 

 a microscopical examination of the rock is 

 required before its place in this classification 

 can be determined — except in a very general 

 way. 



The chemical composition of the rock being 

 known, its mineral composition is first cal- 

 culated. This is readily and quickly done 

 by the aid of the valuable tables appended to 

 the book. Since, however, the same magma 

 may, under different conditions, crystallize 

 out in different mineral combinations, a cer- 

 tain clearly defined method is followed in 

 these calculations, giving that grouping of 

 minerals which the magmas on cooling usu- 

 ally develop. This percentage mineral com- 

 position of the rock expressed in these stand- 

 ard minerals is called the norm, a mineral 

 composition which the magma would normally 



