474 SCIENCE PROGRESS. 



to a very recent date. Microscopic investigation has also 

 detected the important part played in various rocks by such 

 minerals as nepheline, melilite, and the rhombic pyroxenes. 

 With the more complete knowledge of the actual mineralo- 

 gical constitution of igneous rocks thus gained, a compre- 

 hensive classification on this basis became for the first time 

 possible, and this is accordingly a prominent element in 

 most current systems. The application of such systems to 

 the great variety of rocks now described and studied has, 

 however, brought out other difficulties, which are evidently 

 inherent in the method. 



The choice of characteristic minerals has been deter- 

 mined, in the first place, by the important rble filled by the 

 felspar group in the great majority of igneous rocks ; and, 

 in the hands of Roth, Zirkel, Rosenbusch, and their fol- 

 lowers, the dominant kind of felspar in a rock has come to 

 be a prime factor in fixing its classificatory position. Here 

 the need of some ready practical method of discriminating 

 the several felspars has presented an obstacle which is only 

 gradually being surmounted. The presence in some rocks 

 of free silica in addition to acid felspars and the poverty or 

 absence of any felspathic constituent in other rocks have 

 been used to mark further divisions, while the coming in of 

 the " felspathoid " minerals, leucite and nepheline, affords 

 another character of the same rank. Excepting perhaps 

 olivine, the ferromagnesian minerals have been reserved to 

 mark subdivisions of a lower order. 



With whatever degree of success such a scheme may 

 represent the true relationships of rocks of granitoid struc- 

 ture, it can be applied only in a modified form, and not 

 without caution, to the porphyritic types. This point has 

 been emphasised by Michel Levy (23) in an elaborate cri- 

 tique of the second edition of Rosenbusch's Manual. He 

 maintains the propriety of taking as a starting-point in 

 mineralosfical classification the dominant "white element" 

 (felspar or felspathoid) of the second period of consolidation 

 of the rock, rather than the larger and more prominent 

 crystals of the first period, which often make up but a small 

 proportion of the total bulk. He claims to show that the 



