230 GEOLOGY OP THE YELLOWSTONE NATIONAL PARK. 



above the timber is gabbro, which here attains its highest degree of crystal- 

 lization. The outline of the gabbro mass is not well defined against the 

 surrounding breccias, for several reasons. The breccias become highly 

 indurated as they approach the core, and finally appear as dense aphanitic 

 rock, with jointing- planes like those of the finer-grained parts of the massive 

 intruded rocks, and evidences of their originally brecciated character are 

 almost obliterated. Moreover, the gabbro becomes finer grained and darker 

 colored near the encircling rocks, and in some places is so fine grained as 

 to be distinguished with difficulty from the metamorphosed breccia. The 

 shape of the granular core is very irregular and indefinite, for it is found 

 upon investigation that it does not consist of one continuous body of 

 solidified magma, but is made up of smaller bodies differing in grain and 

 mode of crystallization and in mineral composition. Many of these bodies 

 appear as dikes or -veins cutting one another and the larger masses of 

 gabbro. They penetrate the breccia as dikes of crystalline porphyries, 

 whose identity Avith the metre distant, finer-grained dikes is shown by their 

 megascopical habit and mineralogical character. It is evident that in the 

 immediate neighborhood of the heated conduit they cooled at a rate which 

 permitted them to assume a higher degree of crystallization than that 

 assumed by the dikes in the cooler breccias. The transition in grain from 

 the core outward is rapid, and, owing to the variability in the size of the 

 dikes and the differences of crystallization due to this cause, no definite 

 ratio of change was noted. 



Returning to the consideration of the distribution of dikes, it is to be 

 remarked that they abound in the breccias lying north and also east of Cran- 

 dall Creek. In the most easterly mountain of breccia on the map, and on 

 the northwestern spur of Windy Mountain, the breccia of which consists of 

 basaltic scoria and flows, there are numerous dikes, some of which trend a 

 little east of south and others a little west of south. The high limestone 

 escarpment north of Windy Mountain is cut by many small vertical dikes, 

 whose black color contrasts strongly with that of the whitened limestone 

 containing them. In different localities in Crandall Creek where the lime- 

 stone is cut by dikes it is whitened in the same manner in the vicinity of the 

 intruded rock. These dikes also trend east of south, and it is to be observed 

 that there were no signs of a radiation of dikes within the limestone, but 

 the dikes appeared to be located along a system of parallel joints. 



