112 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



coarse-grained varieties with little quartz are lower in silica than the fine- 

 grained varieties with little quartz. (Compare Nos. 297 and 298 with Nos. 

 272, 273, and 274.) It is, of course, evident that in rocks with variable per- 

 centages of the essential minerals which are all silicates there can be no 

 rigid relation between the proportion of any one of these minerals and the 

 silica percentage of the rock within the narrow range of chemical variation 

 that occurs in this group. In it the silica does not vary 7 per cent, and the 

 amounts of the other chemical constituents are the modifying chemical 

 factors. This will be discussed more fully when the chemical composition 

 of the rocks is considered. 



The most regular variation is in the relative proportions of pyroxene 

 and hornblende. There is a definite increase in the amount of hornblende 

 and decrease in that of pyroxene as the rock becomes coarser grained. 

 This is specially noticeable in those specimens forming series from one spot, 

 Nos. 268, 269, 270, 271, 279, and 287, and Nos. 277, 278, 281, 284, and 289. 

 The variation in the relative amount of biotite is not so marked, but there 

 is a slight increase from the fine-grained to the coarse-grained end of the 

 series. 



The irregularities in the variations of the different minerals could be 

 better understood if the chemical composition of all of the different varieties 

 of the rocks were known, but such an investigation is not practicable. The 

 rocks of this subgroup may be classed among the pyroxene-diorites and 

 quartz-pyroxene-diorites. They carry considerable biotite, and pass into 

 quartz-mica-diorite at one end of the series and into pyroxene-porphyrite at 

 the other. 



Tables VI and VII include those varieties of rock in which the amount 

 of feldspar and quartz together exceeds that of the ferromagnesian silicates, 

 Table VII including those varieties particularly rich in quartz. 



The silica percentage is considerably higher in these rocks than in those 

 of the previous subgroup. The quartz is more uniform, and on the whole 

 is higher. It is very considerably higher in Subgroup lie. Pyroxene is 

 absent from most of the varieties, but occurs in small amounts without 

 hornblende in a few instances already noticed. Biotite is more variable in 

 Subgroup lib than in lie, where it is the predominant ferromagnesian silicate. 

 The relation of quartz, biotite, hornblende, and pyroxene to the chemical 

 composition of the different varieties of this series of rocks is not so definite 



