OLD RED SANDSTONE VOLCANIC ROCKS OF SHETLAND. 



375 



On the south-west shore the dip of the strata varies from S.S^E. to S.E. The 

 indurated flagstones and shales pierced by the rhyolites are frequently coated 

 with a green chloritic or serpentinous substance. The dykes are of variable 

 breadth, some of them measuring 12 feet across, while others are considerably 

 in excess of this amount. Generally they preserve a course more or less 

 parallel with the lines of bedding of the flags and shales, but not infrequently 

 they pass transgressively across the edges of the flags. In one instance, the 

 dyke enclosed lenticular patches of altered shales, which were injected with 

 thin branching veins of rhyolite, varying from a few inches to 2 feet in 

 breadth. When traced northwards, this mass rises along the edges of the 

 shales on the cliff. Not far to the north of this locality similar evidence 

 is obtained on the shore of the intrusive character of the rhyolites. The fol- 

 lowing ground plan exhibits the relation of the intrusive dykes to the Old Red 

 strata on this part of the shore. 



At Aith Ness some of the dykes exhibit in a less perfect form the banded 

 structure of those just described, and in this case also they occur close to the 

 north and south boundary fault. It is possible that in some cases this fluxion 

 structure might be partly due to shearing, which might have caused subsequent 

 rearrangement of the materials in connection with the faulting and metamorphism 



Hardened sandstone 

 and flagstone. 



Ml 



Rhyolite. 



Shingle. 



t 1* 



> •_■ Direction of dip of strata. 



Fig. 12. — Ground Plan of rhyolite dykes intruded among Lower Old Red Sandstone strata, south-west shore 



of Papa Little, Shetland. 



of the beds ; but we shall now point out that the same feature is observable in 

 the dykes near Loch Skeld, about 3 miles from the bounding fault. In the 

 Laxa Burn, to the south-west of Bixetter Voe, a dyke of rhyolite was observed, 

 which, on the roadside near the cottage as well as in the burn below the road, 

 merges into a crystalline granitoid rock with quartz, felspar and mica, Again, 

 in the neighbourhood of Loch Skeld, near the granite boundary, we found typical 

 examples where the dykes display fluxion structure as perfect as in Papa Little. 

 The three preceding types of intrusive veins belong to the acidic series, and 

 closely resemble each other in chemical composition, as may be seen at a glance 



VOL. XXXII. PART II. 3 O 



