SYSTEMATIC PETROGRAPHY 4 59 



mineral grains, these authors proceed to give a strangely arti- 

 ficial weight to the products of the second period, both in 

 definitions of structure and in classifying rocks by mineral 

 composition. The structures of eruptive rock applied in classi- 

 fication are brought under two groups : "structures granitoides" 

 and "structures trachytoides." The essential difference between 

 the two is conceived to be that in the granitoid the grains of the 

 two periods of consolidation resemble each other, because of 

 similar conditions of consolidation in the two periods, while in 

 the trachytoid structure there is a marked difference between 

 the two products as a result of changed conditions in the later 

 period. In the rocks commonly called granular it is thought 

 that two generations of mineral grains of approximately the 

 same formal character may usually be recognized. Where no 

 distinction can be made it is rather paradoxically assumed that 

 the grains all belong to the second period. But it is to be noted 

 that without this assumption the scheme of Fouque and Michel- 

 Levy, as it stands, could not classify such a rock. Porphyries 

 in which the groundmass is granular are, from that fact, classed 

 with perfectly granular rocks. 



Under the granitoid group of structures three varieties are 

 recognized: (i) Granitoid proper, in which each individual 

 grain has approximately equal dimensions in all directions, vari- 

 ation in size being disregarded; (2) pegmatoid, the regular or 

 graphic intergrowth of two minerals of simultaneous crystalliza- 

 tion ; (3) ophitic, characterized by elongated feldspar crystals, 

 and forming a transition to the microlitic structure. 



The trachytoid group of structures has likewise three varieties: 

 (i) Type pe'trosiliceux, characterized by bands of minute spheru- 

 lites and the presence of the mysterious substance petrosilex or 

 microfelsite; (2) type microlithique, characterized by microlites 

 of feldspar, and of other minerals ; (3) type vitreux, character- 

 ized by predominance of amorphous substance. 



In explaining the microlitic type the authors point out that 

 their synthetic experiments prove that such microlites are prod- 

 ucts of pure igneous fusion, indicating to them a fundamental 



