OLD RED SANDSTONE VOLCANIC ROCKS OF SHETLAND. 373 



which has taken place. It is evident that the sandstones must originally have 

 covered the whole of the sheet, and may probably have been continuous with 

 the sedimentary deposits of the Mainland. But though the evidence now 

 adduced shows that the felsite has been injected among the members of the 



fiiMilA/fe^ 



Fig. 11. — General section across Papa Stour. 1, Red sandstones and flags ; 2, purple diabase- porphyrites ; 



3, great sheet of pink spherulitic felsite. 



Old Red Sandstone, there is an important lithological difference between the 

 Papa Stour rock and the granitoid masses of Sandsting and Rooeness Hill. 

 This divergence will be discussed when we come to treat of their microscopic 

 characters. 



2. Dykes and Veins. — One of the most interesting features connected with 

 the great intrusive sheets is the number and variety of the dykes and veins 

 radiating from them as centres. Along the boundary of the Northmavine sheet 

 from North Rooe southwards by Hillswick, Mavis Grind, to Aith Voe in 

 Aithsting, these dykes occur in great numbers. Throughout that wide area 

 they traverse the ancient crystalline rocks, the diorite and the altered Old Red 

 strata. In addition to this, we find thin veins and bosses of the same materials 

 penetrating the interbedded volcanic rocks between Brei Wick and Stenness. 

 This latter point is of some importance, as it shows that the intrusion of the 

 sheet, with its branching veins, was subsequent to the ejection of the basic 

 lavas and tuffs. In many cases it is possible to trace the branching veins till 

 they coalesce with the parent sheet ; but more frequently they are completely 

 isolated at the surface, though it is probable that a subterranean connection 

 may exist. The same phenomena are repeated in the case of the great mass in 

 Sandsting and in a less conspicuous form in connection with the Papa Stour 

 sheet. The dykes and veins belonging to the acidic series may be grouped 

 under three divisions — (1) Binary granites and ordinary ternary granites, (2) 

 Quartz-felsites, (3) Rhyolites. In the case of the first group, the dykes still 

 preserve the lithological character of the parent sheet, being coarsely crystalline 

 and consisting mainly of pink orthoclase felspar and quartz with hardly any 

 mica. Some characteristic examples occur in Northmavine, more especially 

 along the coast-line from Mavis Grind to Hillswick. Indeed, the isthmus at 

 Mavis Grind exhibits a curious network of these granitoid dykes interlaced 

 with the diorite. The quartz-felsites are still more numerous than the preceding 

 group. Possessing a fine-grained ground mass with quartz, in distinct crystals 

 or amorphous forms, they are readily distinguishable from the granitoid type. 

 They are typically developed on the shores of Gruting Voe in Sandsting, in 



