THE CLASSIFICATION OF IGNEOUS ROCKS. 475 



grouping arrived at by Rosenbusch is, in many respects, 

 arbitrary, and fails to present a proper correspondence be- 

 tween the families of the porphyritic and the granitoid 

 rocks ; while the classification proposed by Fouque and 

 himself in 1879, which he reproduces with little modifica- 

 tion, is much truer to nature (15). It may be remarked 

 that the petrologists of the French school have endeavoured 

 more systematically than the Germans to carry out the dis- 

 tinctions based on the different kinds of felspar, as, for 

 example, in the separation which they make between ande- 

 sites and labradorites, the one characterised by andesine, 

 the other by labrador felspar. The common use by Ger- 

 man and English petrographers of the name " plagioclase," 

 covering such widely different minerals as albite and anorth- 

 ite, has probably done much to retard the recognition of 

 essential differences among rocks thus loosely thrown to- 

 gether ; and we may endorse the hope expressed by Becke 

 and Michel Levy that methods of increased precision may 

 drive this vague term out of geological literature. Meanwhile, 

 the division into alkali-felspars and soda-lime-felspars, with 

 such subdivision of the latter as is practicable, must be pre- 

 ferred to the distinction foundedoncrystallographiccharacters. 

 The classifications of igneous rocks in current use take 

 account not only of the constituent minerals but of their 

 mutual relations, i.e., of the structural characters of the 

 rocks. An arrangement constructed on this two-fold basis 

 may indeed claim to give some approach to a natural 

 system founded on genetic principles ; since the mineral- 

 ogical constitution of a rock represents in some degree the 

 nature of the parent magma, while the structure affords 

 some index of the conditions of consolidation. In the older 

 systems structural features of comparatively trivial im- 

 portance were often elevated to a prominent rank, and no 

 distinction was observed between original and secondary 

 characters ; so that, for instance, spherulitic and amygdal- 

 oidal structures might be included together under the head 

 of " variolite ". Even in some modern text-books a strict 

 fidelity to purely descriptive characters sometimes leads to 

 anomalies from the genetic point of view. 



