368 



B. N. PEACH AND J. HORNE ON THE 



w 



CI iff Hi Us 



menta discharged from the volcanoes of the period are still preserved to us. 

 From the manner in which they are truncated by faults in Northmavine and 

 overspread by intrusive felsite in Papa Stour, we are justified in inferring that 

 their original limits must have been much greater than now. But the proofs 



V/art of Bressay 



Fig. 5. — Generalised section across the Old Ked Sandstone on the east side of Shetland, showing the position 

 of the volcanic rocks in Bressay. 1, Schists of the Cliff Hills ; 3, Brenista flags ; 4, Rovey Head con- 

 glomerates ; 5, Lerwick sandstones : 6, flaggy series of Bressay and Noss ; 7, volcanic rocks ; 

 /, faults. 



of volcanic activity are not confined to those materials which were accumu- 

 lated simultaneously with the sediment on the sea floor. The phenomena con- 

 nected with the intrusive igneous rocks furnish even more striking proofs of 

 the display of volcanic energy which characterised that period. 



B. Intrusive Igneous Rocks. 



These may be grouped in three divisions — (1) Sheets, (2) Dykes, (3) Necks. 

 Of these divisions the first is the most important, as the sheets cover extensive 

 areas in Northmavine and Sandsting on the Mainland, nearly the whole of Papa 

 Stour, the greater portion of Meikle Rooe and a portion of the island of 

 Vein entry. An interesting feature connected with these intrusive sheets is 

 the evidence which they furnish of the vast amount of denudation which has 

 taken place since Old Red times. It is only by working out the physical 

 relations of these intrusive masses that we can form an approximate idea of 

 the extent of this denudation. 



By far the largest area occupied by these intrusive rocks is in Northmavine, 

 where they extend from the northern headlands of the Mainland opposite Uya 

 Island to Rooeness Voe and onwards to the Heads of Grocken, near Hills- 

 wick. The isolated columns of the Drongs are composed of the same intrusive 

 rocks and likewise the eastern part of Meikle Rooe and the northern portion 

 of Vementry. It is highly probable that the masses just indicated, though 

 now isolated from each other, once formed parts of the same intrusive sheet. 

 Lithologically, as well as microscopically, the rocks bear a close resemblance to 

 each other. The length of this sheet, when measured from its northern limits 

 to Vementry, is about twenty miles, and its breadth in Northmavine varies 

 from three to four miles. This mass is brought into conjunction with the 



