132 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The microstructure of the acid varieties is not the same as that of 

 the basic, so that it is difficult to compare the grain of one directly with 

 that of the other; but since the intermediate rocks possess microstructures 

 intermediate between these extremes, it is possible to establish a kind of 

 relationship between them, and it is admissible to place them in the same 

 line across the table, it being' understood that the correspondence is an 

 approximation. 



A glance at Table XIII shows that a large majority of the varieties 

 are very fine-grained forms that have only readied the crystallization of 

 the few smallest-grained forms of the Electric Peak rocks. A small num- 

 ber of them are more coarsely microcrystalline and correspond to the grain 

 of the dike rocks at Electric Peak. A large number are finer grained than 

 any of these rocks, or are glassy. The coarsest-grained forms have been 

 attained by the most basic varieties, but they do not represent bodies of 

 any considerable extent. Specimen No. 430, grade 25, comes from a small 

 exposure with no definite limits, surrounded by much finer-grained rocks. 

 It is properly a diorite-porphyry, and carries much biotite of final consoli- 

 dation, which has not been reckoned with the phenocrysts. The coarsest- 

 grained forms of the acid varieties, however, represent larger bodies and 

 are more abundant in the field. 



In explanation of the degrees of crystallization indicated in the table, 

 it may be said that the first three are glassy groundmasses, the first one 

 having fewer microlites than the second. In the third the microlites are 

 closelv crowded together. The next two represent microlitic structures in 

 which no glass can be detected; they appear to be holocrystalline. In the 

 sixth grade the form of the microlites is more indistinct, but the general 

 structure is the same as before. Beyond this the different degrees indicate 

 increasing grades of a structure which may be described in general as 

 follows: Commencing with the lowest order, the groundmass is composed 

 of a multitude of indistinct microlites of lath-shaped feldspars; between 

 crossed nicols this aggregation extinguishes light in small patches which 

 bear no fixed relation to the position of the microlites within them. As the 

 dimensions of the lath-shaped feldspars become larger it is observed that 

 the patches of light and darkness arise from the cementing material between 

 these feldspars. This cement possesses the same optical orientation for 

 small spaces which in cross section produce the patches just alluded to. In 



