82 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



in microscopic phenocrysts, and are undoubtedly dacite. No attempt has 

 been made to distinguish on the map the different kinds of rocks forming 

 the large mass and intrusive sheets just described. 



DIFFERENTIATED SHEET SOUTHEAST OF ELECTRIC PEAK. 



One intrusive sheet occurs in the upturned shales of the southeastern 

 spur of Electric Peak which deserves special mention. It is about 30 feet 

 thick, and at present stands in a nearly vertical position, as do the inclosing 

 shales. It is notable on account of its composition and the strongly con- 

 trasted character of the lower and upper parts of the sheet. The rock is 

 massive and greenish near the eastern wall, which was originally the bottom 

 surface; it is fissile and crumbles upon weathering, giving rise to a narrow 

 gulch. Immediately in contact with the shale it is dense, with a purplish 

 tinge. A layer of the sheet, 4 or 5 feet thick, near what was the bottom, is 

 full of large porphyritical augites (225, 228). The remainder of the sheet 

 does not contain them, except sporadically, and carries small feldspar phe- 

 nocrysts. It is more massive, and weathers quite differently from the 

 coarsely porphyritic part. From the abundance of large augite crystals in 

 the bottom portion of the sheet it appears that these crystals must have 

 settled to the lower part while the magma was quite liquid, a phenomenon 

 not observed in any other rocks of this region, where the phenocrysts are 

 uniformly disseminated through the rock. Such separations by gravity 

 have been noted by Charles Darwin 1 in the basalt lavas of the Galapagos 

 Islands, and by Clarence King 2 in the basalt flows of Hawaii. In each of 

 these cases the rocks in which this phenomenon has been observed are 

 basic, as is the intrusive sheet in question. A high degree of liquidity after 

 the phenocrysts have been formed seems to be a necessary condition, and 

 is one most likely to occur in basic rocks. 



The chemical composition of the two parts of the sheet is shown by 

 the following analyses. No. 1 is of the lower portion, crowded with large 

 augite crystals, the layer being about one-sixth of the total depth of the 

 sheet; No. 2 is of the more feldspathic portion; No. 3 is the average 

 composition of the sheet, reckoning the parts analyzed in the proportion of 

 1 to 5. 



1 Volcanic Islands, London, 1851, p. 117. 



2 U. S. Geol. Expl. Fortieth Par., Vol. I, Systematic Geology, p. 715. 



