OLD RED SANDSTONE VOLCANIC ROCKS OF SHETLAND. 371 



localities, where, through the disappearance of the mica, it resembles the binary 

 g anite of Rooeness Hill. The upper limit of the sheet is clearly defined by 

 the altered Old Eed strata between Gruting and Selie Voes. Along this line 

 the observer cannot fail to note how the slope of the upper surface of the 

 intrusive sheet coincides with the angle of inclination of the sedimentary rocks. 

 This feature is admirably displayed on the east bank of Gruting Voe, at the 

 foot of Cullswick Hill. The hardened quartzose flags dip to the north at an 

 angle of about 20°, which is about the inclination of the boundary line between 

 the two. When followed along the junction line, however, the granite cuts 

 across the successive beds, resembling, as we shall see, the behaviour of the 

 sheet of pink felsite on Papa Stour. But in addition to this fact, it frequently 



Fig. 8. — Junction of granite and altered Old Red Sandstone strata in Gruting Voe. 



happens that the altered Old Eed strata and the intrusive sheet are firmly 

 welded together along the junction. In some instances it is possible to dislodge 

 fragments a few square inches across, showing the junction between the two— 

 a phenomenon which conclusively points to the intrusive character of the mass. 

 There is no passage from the sedimentary rocks into the granite, indicating a 

 probable metamorphic origin. On the contrary, the junction line is sharply and 

 clearly defined. This feature, which is characteristic of the Rooeness and 

 Sandsting sheets, is thus referred to by Dr Heddle — " Holding firmly to the 

 view which regards much of the granite in Scotland as the completion of the 

 metamorphism of the gneiss, I am unable to adopt for a moment any such view 

 as regards the granite of Shetland. The nature of the rock itself, the abutments 

 of the stratified rocks against its flanks and the disturbances it has produced 

 among their adjacent layers forbid such a view." * 



The island of Papa Stour is covered for the most part with a sheet of 

 pink spherulitic felsite, forming noble cliffs which rival those of Rooeness Voe 

 and the Heads of Grocken. The same columnar structure is everywhere 

 apparent, not only in the island itself, but in the outlying stacks and skerries, 

 which have been severed from the main sheet by denudation. Fortunately the 

 evidence is sufficiently clear to show that this sheet must have been injected 

 between the underlying diabasic lavas and tuffs and an overlying series of 

 sedimentary deposits, only a fragment of which has escaped denudation. In his 



* Mineralog. Mag., vol. ii. p. 160. 



