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



appear more sombre than they really are when contrasted with the bright 

 red cliffs of granite on the opposite side of the Voe. Some of the beds are 

 highly involved, and show clearly how the partially solidified crnst has been 

 caught up and rolled forwards in the advancing current of the still molten 

 lava. 



From the evidence afforded by these sections, it is manifest that in the 

 peninsular tract of Northmavine, west of Hillswick, there is an important 

 development of ancient lavas and tufts, which attain a thickness of not less than 

 500 feet. The absence of any intercalations of sandstones, flagstones or shales, 

 save near the fault at Brei Wick and Rooeness Voe, is also a feature worthy of 

 notice, as indicating that the subaqueous eruptions must have been well-nigh 

 continuous for a time in that portion of the basin. 



The interbedded volcanic rocks which now fall to be described, occur in the 

 midst of a great series of sedimentary deposits, in the peninsular tract of 

 ground lying to the west of Weisdale. These sedimentary deposits have under- 

 gone so much alteration that Hibbert regarded them as forming part of the 

 metamorphic series, though he noticed that the strike of the former was dis- 

 cordant with that of the latter. This classification was adopted till the summer 

 of 1878, when in the course of our investigations we stumbled on a rich 

 assemblage of plant remains in the beds north of Walls. Many of the speci- 

 mens were badly preserved, but some of them were sufficiently distinct to 

 permit of identification. Upwards of twelve specimens of Lepidodendron 

 nothum, Unger and several examples of Psilophyton princeps, were obtained — 

 forms which are typical of the Old Red Sandstone as developed in other parts 

 of Shetland, Orkney, Caithness, and other regions. There can be no doubt, 

 therefore, that these altered strata, which with the associated volcanic rocks 

 cover an area of about 50 square miles, really form an important development 

 of the Old Red Sandstone of Shetland. While engaged in mapping the bound- 

 aries of these altered strata, we detected certain lavas and tuffs which are 

 regularly intercalated with the series ; the former, however, differing consider- 

 ably from their representatives in Northmavine. From their microscopic 

 characters, as well as from their chemical analysis, it is apparent that the lavas 

 have shared to some extent in the partial metamorphism of the area. But, 

 before defining the localities in which they occur, it may be desirable to indicate 

 their probable geological position. 



Disregarding minor folds, the altered strata seem to form a great synclinal 

 trough, the axis of which runs approximately from Fontabrough Voe on 

 the west coast, east by the village of Walls to the head of Bixetter Voe. 

 On the northern side of the syncline we have a gradually ascending series 

 exposed on the coast from the cliffs of Sandness Hill southwards towards Fonta- 

 brough Voe. Similar ascending sections are to be met with on the shores of 



