:374 B. N. PEACH AND J. HOENE ON THE 



contact with the granite mass, and in the altered Old Red strata at some 

 distance from the main sheet. 



By far the most interesting of these intrusive dykes, however, are the rhyo- 

 lites, which are conspicuously developed in the small island of Papa Little, 

 on the neighbouring shore of the Mainland, and also round the granite mass of 

 Sandsting between Bixetter Voe and Loch Skeld. Possessing a fine-grained 

 semi-vitreous texture, they exhibit in hand specimens a characteristic banded 

 arrangement of the partially devitrified felsitic matter. The constituent bands 

 sometimes display slight variations of tint, which serve to make them more dis- 

 cernible by the eye. This banded arrangement, which, as we shall presently 

 show, is due to fluxion structure, maintains a constant direction in the case of 

 the dykes in the south-east corner of Papa Little. Even when the main dyke 

 sends branching veins into the adjoining strata, the lines of flow have the same 

 trend in the offshoots as in the parent dyke, — a feature which is interesting 

 and suggestive. The rhyolites are usually of a pale grey or yellowish colour, 

 but some of them possess a marked pink or flesh-red tint. This interesting 

 series was detected by us in the course of our third visit to Shetland when 

 tracing the faults bounding the altered Old Red strata west of Weisdale. 

 A subsequent visit was paid to the islands solely for the purpose of determining 

 whether they might represent acidic lavas which had been ejected at the sur- 

 face. The microscopic examination plainly showed that some of the dykes 

 possess fluxion structure in a remarkable degree, and we were therefore anxious 

 to learn whether this might be due to extravasation at the surface and the 

 rapid consolidation of the glassy magma. During this visit our observations 

 were mainly confined to the magnificent sections round the southern shores of 

 Papa Little, where the strata consist of well-bedded but considerably altered 

 flags and shales belonging to the Old Red Sandstone. The two great faults 

 bounding the altered Old Red area in the Mainland cross each other in this island, 

 the one trending towards the north in the direction of Meikle Rooe, while the 

 other preserves an E.N.E. direction. Only the south half of the island is made 

 up of strata of Old Red age, the remainder being composed of gneissose rocks 

 belonging to the series which is so well developed in the centre of the Mainland. 

 The rhyolite dykes are most strikingly represented on the south-east shore, 

 northwards to the point where the fault brings the Old Red strata into con- 

 junction with the metamorphic series. The altered flagstones have a persistent 

 strike about N. 10° W., which is more or less parallel with the north and south 

 bounding fault, and they dip about W. 10° S. at angles varying from 70° to 80°. 

 This strike is quite abnormal, however, and is evidently caused by proximity 

 to the great north and south fault, for when we cross to the south-west shore 

 of Papa Little the strike of the flagstones is E.N.E., which is identical with 

 the trend of the altered Old Red strata over much of the area west of Weisdale. 



