490 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



The ground-mass of the rock varies in structure from the microcrystalline 

 to the devitrified-glassy. Microlites of basic andesine and of augite, with 

 grains of magnetite and apatite, are the determinable original minerals, but the 

 ground-mass is generally altered to the usual obscure mass of secondary chlorite, 

 calcite, etc. 



In spite of the alteration it seems possible to recognize two original, closely 

 related types. One of these, the commoner, is normal augite andesite (specific 

 gravity, 2-673); the other is a hornblende-augite andesite with dominant augite. 



About six miles east of the Pasayten river and 2,500 yards south of the 

 Boundary line the Remmel batholith is interrupted by a circular, pipe-like mass 

 cf volcanic agglomerate about 350 yards in diameter. This rock is quite similar 

 to that at the river, excepting that the breccia here is somewhat coarser, blocks 

 two feet in diameter being common, and, secondly, that, besides the lava-blocks, 

 it carries a large proportion of angular fragments of the Remmel granodiorite. 



The lava of the blocks is often vesicular. It consists of altered andesites 

 apparently belonging to the same two species as those above noted at the river. 

 There is much probability that the two masses of agglomerate are contemporane- 

 ous and genetically connected. 



Two possibilities are open. The small, eastern body may be a part of the 

 cnce continuous volcanic cover locally down-faulted and thus preserved against 

 erosion at the higher level; or the smaller body may occupy one of the actual 

 vents through which the Pasayten andesites were ejected. The rounded ground- 

 plan of the eastern body, its greater coarseness of texture and the abundance 

 of granitic blocks of evidently local derivation, all suggest that the second inter- 

 pretation is the correct one. If so, we have here a volcanic neck, a type of igne- 

 ous-rock body which is by no means common in the northern half of the 

 Cordillera. 



Lightning Creek Diorite. 



On the divide between the south and main forks of Lightning creek and 

 thus from two to three miles west of the Castle Peak stock, the upturned 

 avgillites of the Pasayten formation are cut by two intrusive masses of diorite. 

 The map (No. 14) shows the ground-plan of these two bodies. Each is 

 elongated and both of them cross-cut the sedimentary rocks after the manner of 

 true stocks. The more easterly body is dike-like, being about seven times as 

 long as it is broad. The other body is nearly of the same length, 1-5 miles, but 

 is broader, with a maximum width of 0-6 mile. From the evident similarity of 

 their lithology and of their geological relations, it is reasonable to suppose that 

 the two bodies are connected underground. If so, they represent the partially 

 denuded top of a considerable stock with a roof of irregular form. The total 

 area of diorite as exposed is a little more than a square mile. 



Both bodies have been somewhat, though not greatly, squeezed and sheared, 

 so that the diorite generally has a gneissic structure. The secondary planes 

 strike on the average about N. 40° W. and their dip is about vertical. In con- 

 sequence of this dynamic action the diorite is notably more altered than is the 



