OLD RED SANDSTONE VOLCANIC ROCKS OF SHETLAND. 



metamorphic series at the Heads of Grocken by a fault, which is admirably 

 seen on the shore. 



In Meikle Rooe the granite mass is also faulted against the older rocks, 

 which consist mainly of diorite with occasional patches of mica schists. In 

 all likelihood, the fault at the Heads of Grocken and in Meikle Rooe is merely 

 the northern prolongation of the great north and south dislocation bounding 

 the altered Old Red strata west of Weisdale. On the shores of Rooeness Voe, 

 however, and northwards by the Biurgs, on the eastern seaboard of North- 

 mavine, the granite spreads over the ancient crystalline rocks in the form of a 

 great sheet, without deflecting the strike of the metamorphic series, and termin- 

 ates along the eastern margin in a great escarpment 200 feet high. The North- 



Fig. 6. — Binary granite faulted against the ancient crystalline schists, Heads of Grocken, Northmavine. The 

 Headlands of Stenness, and the Islet of Doorholm in the distance, formed of bedded lavas and tuifs. 



mavine mass consists mainly of a binary granite or aplite, composed of quartz 

 and pink orthoclase felspar, shading occasionally into salmon-coloured quartz - 

 felsite. Generally the rock is coarsely crystalline and highly siliceous, and 

 there can be no doubt that the mass must have consolidated under great 

 pressure, though the materials under which it lay buried have been wholly 

 removed by denudation. The presence of so much silica has no doubt retarded 

 the general denudation of the Rooeness plateau, but it has been ineffectual in 

 preventing the waste caused by the sea. But apart from the coarsely crystalline 

 character of the rock, the marked columnar structure suggests the idea that it is 

 a great intrusive sheet which has consolidated underneath the surface. Those 

 who wish to study this feature would do well to sail down Rooeness Voe or 

 along the shores of St Magnus Bay from the Heads of Grocken to Brei Wick 

 (see Sketch of Heads of Grocken, fig. 6). Along the cliffs the observer is 

 confronted by symmetrical columns rising from the sea-level, which are tra- 



