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



The beds surrounding the necks consist of red sandstones, which are much 

 shattered and baked along the lines of junction with the agglomerate. They 

 dip seawards (towards the east) at angles varying from 20° to 25° ; but towards 

 the edge of the fissure they are highly inclined. When the sandstones are 

 followed inland, the alteration which is so apparent at the line of junction 

 gradually disappears. As may .be seen from the accompanying ground plan 

 (fig. 14), the outline of the vents is very irregular. 



The materials filling the vents consist of a coarse agglomerate, made up of 

 angular fragments of sandstones, flags and shales, imbedded in a finely- 

 comminuted paste. Occasionally large masses of the surrounding sandstones 

 are enclosed in the agglomerate, which are highly crystalline. Besides these, 

 there are masses of red calcareous and highly-baked muclstones, which have 

 been torn from the sides of the vent. No bombs of porphyrite occur in the 

 agglomerate, — at least, none was observed by us. A vein of porphyrite a few 

 inches thick is traceable along the margin of the orifice for a short distance, 

 and coatings of a diabasic lava occur on the surfaces of the indurated sandstones. 

 We also noticed a thin vein of copper pyrites traversing the agglomerate and 

 the altered sandstones. 



Crossing the Noss Sound, another neck is visible, which resembles that just 

 described in the nature of the material filling the vent and in its mode of 

 occurrence. Round the neck the gradual folding inwards of the flags near the 

 edge of the agglomerate is worthy of note, as it forms a characteristic feature 

 of the stratified rocks in immediate contact with the Carboniferous volcanic 

 vents in central Scotland. From the nature of this volcanic agglomerate it is 

 highly probable that no lavas were ejected from these orifices. It is more 

 likely that they served mainly as blow-holes, discharging occasionally showers 

 of triturated materials derived from the sides of the vents. 



We shall now briefly summarise the succession of events as indicated by the 

 volcanic phenomena described in the foregoing pages. The earliest ejections 

 consisted of basic lavas and tuffs, which were spread over the sea-floor, and in 

 several instances were- intercalated with the ordinary sediment. In North- 

 mavine the volcanic accumulations were nearly continuous, save towards the 

 horizon of the lowest beds, where flagstones and ashy sandstones alternate with 

 the lavas. In the Aithsting and Sandness district the ejections were limited 

 to a few sheets of diabase and some bands of tuff, which are interbedded with 

 a great succession of altered sandstones, flags and shales. The sheet of 

 porphyrite in Melby Holm indicates a recurrence of volcanic activity, which 

 became more pronounced as the conglomerates, sandstones and flags of Papa 

 .Stour were deposited. In the latter case the discharge of volcanic materials 

 must have been intermittent. The band of tuff and the necks in Bressay 



