Messrs Peach & Home on Old Bed Sandstone of SJietland. 87 



On the eastern shore of Bressay, opposite the north end of 

 the island of ISToss, we discovered a bed of tuff, interbedded 

 with the flags, which is probably connected with the vol- 

 canic pipes in that neighbourhood. 



Intrusive Igneous Rocks of Old Bed Sandstone Age. — In 

 North Ma vine, as well as in the districts of D citing, Sand- 

 ness, and Sandsting, on the Mainland, there is a series of 

 intrusive masses, which doubtless belong to this period. 

 These intrusive rocks vary considerably in lithological 

 character, but they all agree in possessing a large proportion 

 of silica, while the felspar is almost invariably orthoclase. 

 The dome-shaped mass of Eoeness Hill is formed of this 

 material, while northwards it extends to the shores of the 

 Mainland, opposite the island of Uya. It likewise crosses 

 the peninsular tract west of Hillswick to the Heads of 

 Grocken, reappearing in the Drongs, and on the west side of 

 Meikle Rooe. From the marked columnar structure which 

 characterises these rocks on the banks of Eoeness Voe, and 

 between the Heads of Grocken and Braewick Bay, as well as 

 from the manner in which the Eoeness mass spreads over 

 the edges of the metamorphic rocks, we are inclined to believe 

 that the granite and quartz-felsite in that district is an 

 intrusive sheet, which was injected between the underlying 

 metamorphic rocks and the overlying Old Eed Sandstone 

 strata, which have been long since removed by denudation. 

 The quartz porphyry of Papa Stour, which covers nearly the 

 whole of the island, is an intrusive sheet injected along the 

 lines of bedding of the Old Eed Sandstone rocks, as numerous 

 sections clearly show. Indeed, a smaU patch of red sandstone 

 is still to be met with resting on the pink porphyry at the 

 Horn of Papa. Though no trace of the once superincumbent 

 strata is now visible on the Eoeness mass, this is not to be 

 wondered at, when we consider the great denudation which 

 has taken place since Old Eed Sandstone times. 



In the district of Sandsting there is satisfactory evidence 

 to prove that the granitic mass between Gruting and Skelda 

 Voes has been injected along the lines of bedding of the 

 altered Old Eed Sandstone rocks. While the inclination of 

 the granite mass is nearly the same as that of the quartzites 



