300 B. N. PEACH AND J. HORNE ON THE 



No previous attempt has been made to furnish a chronological account 

 of the Old Red volcanic phenomena of those northern isles. In Hibbert's 

 admirable volume "" there are various references to the granite masses of the 

 Mainland and the amygdaloidal claystones in the south-west of Northmavine. 

 He also refers to the porjDhyritic and amygdaloidal rocks in Papa Stour, which 

 were likewise described by Dr Fleming^ In various papers published in the 

 Mineralogical Magazine,\ Dr Heddle notes the existence of interbedded and 

 intrusive igneous rocks of this age in Shetland, with descriptions of the minerals 

 obtained from them. The first attempt, however, to connect these Old Red 

 volcanic rocks with their representatives south of the Grampians, was made by 

 Dr Archibald Geikie, the present Director-General of the Geological Surveys. 

 In 1876 the geological structure of Papa Stour, which is almost wholly com- 

 posed of volcanic rocks, was solved by him in company with Mr B. N. Peach '> 

 and, as the result of that traverse, an account of the geology of that interesting- 

 island was given in his celebrated paper on " The Old Red Sandstone of 

 Western Europe," published in the Transactions of this Society. § Though 

 unable to visit the volcanic rocks on the north side of St Magnus Bay, he 

 ventured to suggest that^ the amygdaloidal claystones referred to by Hibbert 

 would turn out to be merely a repetition of those in Papa Stour, — a suggestion 

 which has been amply verified by subsequent investigations. 



During our successive visits to Shetland, which were undertaken mainly 

 with the view of examining the glacial phenomena of the group, we were 

 induced to pay close attention to the distribution and geological structure of 

 the Old Red Sandstone rocks, on account of the important bearing which they 

 have on the ice-carry during the glacial period. A brief sketch of the develop- 

 ment of the contemporaneous and intrusive igneous rocks was given in the 

 paper which we communicated to the Geological Society in 1879. || But since 

 that paper was read we have twice visited the islands in the course of our 

 holiday rambles, in order to work out in greater detail the volcanic history of 

 that period. Our last visit was specially devoted to the investigation of an 

 interesting series of rhyolites, which have not hitherto been described, though 

 at certain localities they have a remarkable development. A large number of 

 microscopic sections have been prepared and examined, while detailed chemical 

 analyses of the typical volcanic rocks have been made for us by our friend Mr 

 R. R. Tatlock, F.R.S.E., one of the public analysts for Glasgow. We now 

 propose to lay the results of these investigations before the Society. 



* Hibbert's Shetland Isles, pp. 341, 474, 484, 491. 



t Mem. Wernerian Soc, vol i. p. 162. 



% Mineralog. Mag., vol. ii. pp. 160, 170 et seq., 253 ; vol. iii. p. 32. 



§ Traits. Edin. Roy. Soc, vol. xxviii. p. 345. 



|| Quart. Jour. Geol. Soc, vol. xxxv. p. 786. 



