VARIATIONS OF TEXTURE IN IGNEOUS ROCKS 603 



In No. I the relation KgO + Na^O : CaO = i : .2. In No. 2 

 the same relation equals i : .53. In the same way, in No. i, 

 K3O : CaO = I : .46. In No. 2 the same ratio equals i : i. 



Conclusions. — In the field the evident relation of the dikes to 

 the rhyolite led to the inference that they had been the feed- 

 ers of the extrusive rock. Under the microscope the composi- 

 tion of the three rocks is found to be the same, and the structure 

 shows variations indicating no great differences in the condi- 

 tions of cooling. The structure is identical with that of certain 

 specimens of rhyolite-granite just described from the Pinenut 

 range, and therefore need not be analyzed again. Briefly, in 

 all three it indicates complete crystallization in one place, with 

 no interrupting movement. The period of consolidation for 241 

 N was comparatively short, that of 243 N somewhat longer, 

 and that of 242 N markedly greater than 243 N. 



ANALOGOUS CASES OF VARIATIONS OF TEXTURE IN OTHER PARTS 

 OF THE GREAT BASIN. 



In the Washoe district, Nevada, not far from the first 

 locality decribed by the writer, Messrs. Hague and Iddings' 

 found a gradual transition from pyroxene andesites with glassy 

 Sroundmass to pyroxene-diabase with coarse granular structure. 

 In the Sutro Tunnel they found coarsening of the crystallization 

 as the tunnel nears the core of Mount Davidson, so that at one 

 end the rock may be called andesite and at the other end diabase. 

 They also discovered like transitions between andesite and 

 granular diorites. 



Similarly they found that the earlier hornblende-andesite 

 passes into diorite, while the later hornblende-mica-andesite 

 changes into mica-diorite in such a way that the two rocks are 

 inseparable. They concluded that a dike of so-called diabase 

 is a variation of the basalt, which was one of the latest extrusions. 



In short, according to these writers, the coarse holocrystalline 

 rocks of the Washoe district are chiefly Tertiary, and are partly 

 extrusive and partly closely connected with extrusives of similar 



'Bull. 17, U. S. Geol. Surv. 



