370 B. N. PEACH AND J. H01INE ON THE 



versed by a series of vertical joints. Hence it follows that the vertical face 

 of the cliff is preserved, though constantly assailed by the sea and subjected 

 to continual recession by the removal of huge slices of rock. Frequently the 

 columns are isolated and left to battle with the denuding agencies as best 

 they may. The columns of the Drongs are beautiful relics of the Eooeness 

 Hill sheet which have hitherto been able to resist complete demolition. 



"We have already referred to the fact that the only place where this intru- 

 sive mass is seen in contact with the interbedded volcanic rocks of North- 

 mavine is on the south bank of Rooeness Voe, where the latter are thrown 

 down by a fault against the former. We have therefore no indication of the 

 thickness of strata which originally covered the plateau. But, from the 

 columnar structure, the coarsely crystalline texture, from the manner in which 

 it spreads over the metamorphic series like a great cake, we have come to the 

 conclusion that the Eooeness mass is an intrusive sheet which forced its way 



Fig. 7. — Section across Northmavine from Ockren Head to Skea Ness. 1, Metamorphic rocks ; 2, serpentine ; 

 3, granite and quartz-felsite ; 4, breccia of serpentine fragments ; 5, bedded porphyrite and tuff ; /, faults. 



upwards and laterally between the metamorphic strata on the one hand, and 

 the members of the Old Red Sandstone on the other, at the time when the Main- 

 land lay buried under the deposits which accumulated during that period. It 

 is right to state, however, that, so far as our observations went, there are no 

 beds underlying this igneous mass which could be referred to the Old Ked 

 Sandstone. At one locality, to the north of Colafirth Voe, a curious brecciated 

 serpentinous mass occurs, which might, on further examination, prove to be a 

 basal breccia of this age. Be this as it may, it is evident that the underlying 

 platform consists mainly of cliorite and various metamorphic rocks. The fore- 

 going conclusion is confirmed, as we shall presently point out, by an examina- 

 tion of the relations which the Sandsting granite mass and the Papa Stour 

 felsite bear to the Old Red strata. 



The age of the granite mass of Sandsting is placed beyond doubt by a study 

 of its relations to the altered Old Red strata of that district. It covers a 

 triangular area, extending from Selie Voe to Gruting Voe. A portion of the 

 sheet is also met with in the south-east of the island of Vaila. When 

 measured along a line from Vaila to Selie Voe, the mass is 6 miles long, and 

 its greatest breadth is about 4 miles. Presenting the same columnar structure 

 as the Rooeness sheet, it immediately suggests the idea of a common origin. 

 Lithologically it approaches the type of an ordinary granite, save at certain 





