366 B. N. TEACH AND J. HORNE ON THE 



arc on the same geological horizon as the Lerwick series, then we are able 

 to fix the date of the ejection of the contemporaneous volcanic rocks in 

 Aithsting and Sandness. The latter are admirably seen on the shores of 

 North and South Clouster Voe, whence they can be followed north-east- 

 wards towards Aith Ness. The volcanic rocks consist in that region of dark 

 green diabase lavas, which only occasionally exhibit vesicular cavities on the 

 surface of the flows. But from this feature, as well as from their perfect 

 parallelism with the sedimentary strata, there can be no doubt of their con- 

 temporaneous origin. In chemical composition they are highly basic, and 

 strongly resemble the diabase-porphyrites of Northmavine, though there is a 

 difference in one important particular, which will be subsequently referred to. 

 Again, on the western shores of Sandness the quartzites and flagstones are 

 associated with bands of tuff, which are exposed on the coast at the mouth of 

 Dale's Voe and further south near Watt's Ness. The volcanic ash can be 

 traced inland in a north-east direction by the Stourbro Hill. The lavas and 

 tuffs now referred to dip to the S.S.E., like the altered strata with which they 

 are associated. They are not exposed on the south side of the synclinal axis ; 

 at least we saw no indications of them in the course of our traverses. Their 

 absence may be accounted for by the occurrence of the great intrusive sheet of 

 granite which was intruded along the lines of bedding at a period subsequent to 

 the ejection of the lavas. 



The small tract of unaltered Old Red strata at Melby contains no inter- 

 bedded volcanic rocks, but in the Holm of Melby — a small island between the 

 Mainland and Papa Stour — there is an interesting relic of volcanic activity. 

 The arrangement of the strata is shown in the following section : — 



i 



Fig. 3. — Section across the Holm of Melby. 1, Sandstones and flags ; 2, porphyrite. 



Along the east coast runs a well-marked anticline of grey flags, which, 

 towards the west, pass underneath a bed of dull purple porphyrite, forming 

 the ridge in the centre of the Holm. This sheet of lava is overlaid by another 

 series of grey flags, strongly resembling some of the flagstones in the Sound of 

 Noss, east of Bressay. Possessing the usual amygdaloidal characters, this 

 bed strongly resembles the type of porphyrites developed in Northmavine. 



Crossing the Sound of Papa to the island of Papa Stour, further evidence 

 is obtained of the intercalation of diabase-porphyrites in the flagstones, sand- 

 stones and conglomerates. The geological structure of this interesting island 

 lias been graphically described by Dr Archibald Geikie in the Transactions of 

 this Society, and little else remains to be added to the account given by him. 



