ABSAEOKITB OF CACHE CHEEK. 335 



The fifth rock of this class, of which the chemical analysis has been 

 given, occnrs as a massive body in the andesitic breccia on the southeast 

 fork of Cache Creek. Another rock of like character forms a narrow dike 

 on the divide between Cache and Crandall creeks (1282, 1283). They are 

 aphanitic, brownish-gray rocks, without phenocrysts, but having minute 

 brown pseudomorphs, presumably after olivine. The rocks are identical in 

 every respect and carry occasional rounded grains of dark-colored, crackled 

 quartz, inclosed in a thin green shell. 



In thin section these rocks are noncrystalline and fine grained, con- 

 sisting of thin lath-shaped feldspars, frequently grouped in fan-like clusters. 

 They are not distinctly striated, and are probably orthoclase. Others are 

 plagioclase with low angles, and some, are irregularly bounded and obscure. 

 This matrix is crowded with idiomorphic crystals of pale-green augite, 

 dark-brown biotite, 1 and magnetite, the ferromagnesian minerals equaling 

 the feldspar in amount. There are a few porphyritical augite groups, 

 which are almost colorless at the center, with pale-green margins. The 

 pseudomorphs have the outline of olivine. The feldspars exhibit slight 

 alteration, but the augites and biotites are perfectly fresh. In places there 

 are ilmenite rods. 



There is considerable serpentine scattered through the rock in small 

 aggregations of pale-green spherulites. Mineralogically the rock is very 

 similar to the quartz-bearing dike rock (1277) from the ridge southeast of 

 Index Peak, which it resembles megascopically. Chemically it is slightly 

 higher in alumina and potash, and slightly lower in magnesia and lime. 

 The high loss on ignition is most likely due to the serpentine, which was 

 probably derived from olivine. 



Fortunately, this phase of the magma is found in immediate connection 

 with the gabbro core on Hurricane Mesa, Crandall Basin. It forms the 

 dense bluish-black margin (1422) of a 4-foot dike, the middle of which is 

 grav and crystalline (1421). The dike cuts granular rock near the base of 

 the middle spur of the core. The aphanitic marginal rock (1422) carries a 

 few small micas, but no prominent phenocrysts. In thin section it has the 



1 When first studied, some of the brown minerals were considered to he brown hornblende, but 

 a careful review fails to establish the presence of hornblende. Some of the biotite plates are long 

 and narrow, and when they overlie a crystal of feldspar they appear to possess a double refraction not 

 found in other biotite plates, which led to their being mistaken for hornblende. The rock was doubt- 

 fully named hornblende-miuette in the list of rocks whose analyses were given in Table III in the 

 paper on The origin of igneous rock, in Bull. Philos. Soc. Washington, Vol. XII, p. 199. 



