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PKOCEEDINGS OY TELE GEOLOGICAL SOCIETY. [June 9, 



while at the opposite point of contact on the S.E. the existing dip 

 might remain comparatively unaffected. The upheaval along this 

 line of fault, vrith which the porphyritic rock (a) is believed to be 

 contemporaneous, probably took place prior to the deposition of the 

 Old Eed Sandstone, which, overlying the Lower Silurian rocks un- 

 conformably,dips generally seawards in accommodation to what would 

 seem to have been a previously elevated coast-line. Throughout the 

 auriferous district the red granite (h) appears to answer the descrip- 

 tion given by Prof. Harkness — an intrusive rock " conforming in 

 its course to the strike of the previously elevated strata." The 

 binary compound of felspar and quartz referred to as granitoid (c), 

 and apparently associated with the richest auriferous drifts, seems 

 in many instances to suggest a metamorphosed Lower Silurian rock 

 whose particles, yielding to such agencies as heat and electricity, 

 were melted and mineralized in situ. It does not appear to me 

 altogether inconsistent with this opinion that the rock in question,, 

 which is as thinly bedded as the flaggy sedimentary strata, and alter- 

 nates with these, whether they be gneissose, quartzose, or micaceous, 

 should occur as a short transverse dyke, as it seems to do at Suisgill 

 (fig. 2). If we suppose that fissures caused by local upheaval 

 might have been contemporaneous with an exhibition of metamor- 

 phic action, these fissures would be filled by a molten rock, whether 

 its materials were supphed on the spot or intruded from beneath. 

 If, however, it can be shown that rocks are never melted by the 

 agencies which conduce to metamorphism, then both b and c seem to 



Fig. 3. — Granite and Trap in Mica-schist, above Crash Bridge, and 

 near Kil^Donnan Lodge. 



1 3 2 2 12 



1. Micaceous Schist, dipping N.E. 2. Granite (6). 



2 1 



3. Trap Dyke. 



be as truly plutonic and intrusive as the trap-dyke at Crask Bridge 

 on the UUie (fig. 3). 



4. Eeferring to the accompanying map of the auriferous district, 

 (Plate XIII.) I beg to offer a few detailed descriptions, beginning on 

 the S.W. at 



Strathbrora. 



The Uisge-dubh, or Blackwater, runs nearly across the strike of 

 highly inclined flaggy quartzites and micaceous schists, whose dip 

 ranges from S. to E. At the waterfall, below Ceann-an-tuir, gra- 

 nite (5) appears among beds of micaceous schist which have been 

 dislocated by a fault. Parallel courses of this rock recur at short 



