﻿1848. SLAVE RIVER. 139 



genous rocks ; and I regretted that I could devote 

 no time to this purpose. The beds of limestone, 

 as seen in passing rapidly along these islands, 

 appeared of various thickness, some being thin 

 and shaly, and almost all more or less undulated, 

 saddle-formed, or contorted. On the borders of a 

 channel between two of the islands, a conglomerate 

 is interleaved with sienite ; and in the vicinity there 

 are beds of a brownish, finely crystalline limestone, 

 having a conchoidal fracture, the fragments being 

 sharply angular. The conglomerate varies con- 

 siderably in its texture in different layers, and 

 even in different parts of the same bed. It con- 

 tains, in general, a large proportion of small 

 rounded grains of translucent or milky quartz, 

 with angular fragments of various sizes of vitreous 

 quartz, chlorite-slate, and calc-spar, imbedded in a 

 powdery or friable white basis, which does not 

 effervesce with acids ; the whole forming a tough 

 stone. In some beds the quartz grains predomi- 

 nate, so as to render the rock a coarse sandstone ; 

 but in other parts, these grains appear to have 

 been fused into a bluish quartz rock, the original 

 granular structure being only faintly discernible, 

 and to be detected chiefly in spots, where some of 

 the powdery basis remains unchanged. In one 

 bed, angular fragments of greenstone encrusted 

 with calc-spar occur. The sienite contains grains 



