VARIATIONS JN GRANITE PORPHYRY. 225 



in color. The lower beds on the west side are darker in color, more sili- 

 ceous in composition, and rich in black cherty nodules, characteristics which 

 everywhere define the upper from the lower beds of the Pogonip lime- 

 stone. 



That the strata near the southwest end of the dike belong- to the upper 

 portion of the Pogonip epoch is evidenced by the patches of Eureka 

 quartzite still in place and by a number of loose bowlders and quartzite 

 debris scattered over the hill slopes. 



In the middle of the main dike, just north of the road which follows 

 the Spring Valley drainage channel, there occurs a curious bit of cherty 

 limestone. It is several hundred yards in length, but only a few feet in 

 width, and lies completely surrounded by granite-porphyry, which may be 

 seen penetrating and filling up the irregular outline in the limestone. In 

 places the molten mass appears to have eaten into the sedimentary body, 

 although only to a very limited extent. Along the contact both the por- 

 phyry and limestone present the same phenomena of cooling as seen near 

 the outer walls of the main dike. Even this narrow body of limestone 

 does not appear to have undergone much metamorphism, except along the 

 contact. 



Structural Variatinos in Granite-porphyry. The chief interest attached tO the 



granite-porphyry lies in the very variable structural differences produced 

 in the erupted material of the dike, differences which are mainly dependent 

 upon the chilling effect of cold contact walls upon a rapidly cooling molten 

 mass. The width of the dikes has much to do in determining these physical 

 conditions governing crystallization. In other words, development of crys- 

 tallization is dependent upon rate of cooling, and in narrow dikes a molten 

 magma is more rapidly chilled than in broader bodies. There are probably 

 few localities in the Great Basin where the results of rapid chilling and 

 crystallization of a granite magma in narrow dikes along several miles of 

 contact walls can be studied to better advantage or are more worthy of a 

 detailed petrographical investigation. For petrographical details the reader 

 is referred to the paper by Mr. Joseph P. Iddings. 



The large oval-sha.ped area to the north and the broader central por- 

 tions of the main dike are quite similar rocks, presenting the characteristics 

 MON.XX 15 



