OLD RED SANDSTONE VOLCANIC ROCKS OF SHETLAND. 



367 



In the course of our subsequent visit we obtained some additional evidence 

 regarding the intrusive character of the sheet of felsite which forms such a 

 prominent feature in the geology of the island. At various points on the coast- 

 line, sometimes at the base of the cliffs, sometimes forming the whole cliff from 

 top to bottom, there are certain red sandstones, flags and conglomerates which 

 are clearly interbedded with purple slaggy porphyrites. That these lavas were 

 ejected during the time of the formation of the sedimentary deposits is proved 

 by the occurrence of blocks of diabase-porphyrite in the conglomerates. 

 Neither the sandstones nor the interbedded porphyrites are traceable for any 

 distance, as they are cut through and overlaid by a sheet of pink felsite, which 

 covers nearly the whole of the island. The lavas occur in Hamna Voe on the 

 south side of the island, on the shore east of the church in Housa Voe, on 

 both banks of Culla Voe, and again on the headlands of Bordie in the north- 

 west of the island. 



The only relic of interbedded volcanic materials on the east side of Shetland 

 is met with on the east coast of the island of Bressay in Noss Sound. Opposite 

 the north end of the island of Noss, on the east side of the fault skirting the 

 shore below Ander Hill, a bed of brecciated tuff about seven feet thick is asso- 

 ciated with grey flags. The strata dip to the east at a low angle and are 

 repeated by small faults, as shown in the accompanying ground plan. 



Fig. 4. — Intercalation of tuff (2) with grey flags (1), S.E. shore of Bressay. 



There can be little doubt that this band of tuff is directly connected with 

 the remarkable series of volcanic necks which occur not far to the south on 

 the same island. When we come to describe the nature of the materials which 

 now fill these vents, we shall see that it is highly probable that no lavas were 

 ever ejected from these orifices. It is more likely that they mark but a feeble 

 development of volcanic energy, during which occasional showers of tuff were 

 spread over the sea-floor. If this be true, then, this band of tuff may be 

 regarded as a relic of this sporadic outburst. Occurring near the top of the 

 flaggy series of Bressay, it is evident that this tuff occupies a high geological 

 position in the order of succession in the eastern seaboard. It is not improb- 

 able, therefore, that it may represent portions of the contemporaneous volcanic 

 series of Papa Stour and Northmavine. 



We have now completed the sketch of the interbedded volcanic rocks, and, 

 from the foregoing descriptions, it is manifest that only portions of the ejecta- 



VOL. XXXII. PAKT II. 3 N 





