VARIATIONS IN GKANITE rUEPHYKY. -22i) 



iu color. The lowor beds oil the west side are darker in color, more siH- 

 ceous in composition, and rich iu blackcherty nodules, characteristics which 

 everywhere define the ujiper from the low^er beds of the Pogonip lime- 

 stone. 



That the strata near the southwest end of the dike beloii<r to the uj)per 

 portion of the Pogouij) epoch is evidenced by the patches of Eureka 

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

 d(^bris 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 liit of cliertv 

 limestone. It is several hundred viirds in length, l)ut only a few feet in 

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

 seen penetrating and filling uj) tlie irregular outline in the limestone. In 

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

 although only to a very limited extent. Along the contact both the })or- 

 pliyry 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 ajjpear to have undergone much metamorphism, except along the 

 contact. 



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



granite-porpliyry lies in the very variable structural diflferences produced 

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

 u})oii tlie chilling eff"ect 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, develojunent of crys- 

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

 magma is more rapidly chilled than in l)roader bodies. There are i)rol)ably 

 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 ))ai)er by Mr. Josejih P. Iddings. 



The large oval-shaped area to the north and the l)roader central por- 

 tions of the main dike are (piite similar rocks, presenting the characteristics 

 MON XX 15 



