108 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



with the mineral composition. But when the size of the dikes in each case 

 is taken into consideration it becomes even more evident than from the 

 table that the coarseness of grain bears a very considerable relation to the 

 chemical composition of the rock. The variation in grain between the sides 

 and center of a dike and between dikes of different widths, for rocks of 

 nearly the same composition, is not so great as the variation between rocks 

 of different composition where the size of the dikes in which they occur is 

 somewhat similar. Thus, specimen No. 233 is from the center of a 4-foot 

 dike, and No. 232 from the contact wall of the same; and specimen No. 247 

 is from the center of an 8-foot dike, and N6s. 257 and 250 are from the 

 contact walls of the same; Nos. 264 and 265 are from 4-foot dikes, and 

 No. 263 is from a 2-foot dike. They all occur at nearly the same altitude, 

 but it is possible that the pyroxene-bearing rock, No. 233, may have been 

 intruded in rocks which were more heated at the time of its intrusion and 

 so have acquired its degree of crystallization through slower cooling, but 

 this is not so likely to have happened in the case of rock No. 234, which 

 is in the same part of the mountain as No. 235, but is in a dike 10 feet 

 wide and is very much coarser grained than No. 233. (See Table VIII.) 



The groundmass of the rock with porphyritical quartz and biotite, No. 

 265, is made up of minute grains of quartz and feldspar, about 0.015 mm. in 

 diameter, while the groundmass of the pyroxene-bearing variety, No. 233, is 

 made up of lath-shaped and irregularly shaped feldspar about 0.10 mm. to 

 0.14 mm. in length, and the groundmass of No. 234 is composed of lath- 

 shaped feldspars 0.5 mm. to 0.7 mm. in length. 



The character of the °'roundmass changes from an even g*ranular 

 structure in the acidic rocks, through one made up of irregular grains and 

 lath-shaped feldspars in the intermediate rocks, to an aggregation of lath- 

 shaped feldspars with almost no irregular grains in the basic varieties. 



The tendency of basic rocks to crystallize more completely and with 

 larger groundmass crystals than acidic rocks is constantly observed among 

 the extrusive rocks, such as basalts, andesites, and rhyolites. The same 

 law appears to obtain among the intrusive rocks. It is of course necessary 

 to compare rocks that appear to have crystallized under very nearly the 

 same physical conditions. 



The rocks of Group II have been described in greater detail on account 

 of their number and importance, and have been subdivided into three sub- 



