646 DESCEIPTIVE GEOLOGY. 



Lima (Clenoides) Gabbi. 

 Ammonites {Gymnotoceras) Blakei. 

 AcrocJiordiceras Hyatti. 

 Eutomoceras Laubei. 

 Lima {Limatula) erecta. 



Of these Professor Meek regards the latter as indicating rather the 

 horizon of the Lower Liassic. 



Along the southeastern edge of the hills is a development of diorite, 

 which also forms the country-rock of the mines at their southern point. 

 This diorite is a dark-green, compact rock, with veinings of lighter green 

 running through it, which, to the naked eye, presents an almost homoge- 

 neous mass. A few large greenish plagioclase-feldspar crystals can be 

 distinguished, and also occasional prisms of hornblende. Under the micro- 

 scope, besides the hornblende and feldspar, magnetite, some apatite and a 

 colorless mineral, which is probably tremolite, are also distinguished. The 

 feldspars show vestiges of former twin striation, and are filled up wit/h fine, 

 needle-like fragments of hornblende, to which they owe their green color. 

 The hornblendes are particularly interesting as showing the peculiar 

 structure which has been figured by Professor Zirkel in his report.^ They 

 are made up of an aggregation of microscopical prisms arranged parallel to 

 each other and in such form as to build up the macroscopical hornblende 

 crystal. 



This diorite body has been covered by the flows of rhyolite, whicli 

 extend high up on to the flanks of the hills, and to the north cover com- 

 pletely the low ridge which connects these mountains with the Augusta 

 Mountains. The rhyolite of this northern ridge is of the red porphyritic 

 variety, very rich, however, in crystals of sanidin and apatite, and having 

 the peculiar banded structure observed in the rhyolite of the ridge between 

 Squaw Valley and Rock Creek Valley, to the north of the Humboldt River, 

 in whicb the bands consist of alternately predominating portions of a reddish 

 felsitic groundmass and of aggregations of crystals of sanidin and quartz. 



Immediately adjoining the diorite is a peculiar rhyolitic breccia of light- 



^ Microscopical Petrography, vol. vi, 88, and Plate I, fig. 11. 



