﻿272 OBIGIN OF THE PILLOW-LAVA IN CORNWALL. [May I908, 



whole, much larger than those of the pillow-lavas, suggesting that 

 the latter were formed under considerably greater pressure. That 

 the lavas were free to move was clear, and it seemed that the com- 

 bination of freedom and pressure could be best obtained on the floor 

 of a tolerably-deep sea, as the Authors suggested. The amount of 

 occluded gas that could be present in a perfectly-solid rock had 

 been shown by Mr. Harker in the Geological- Survey Memoir on 

 Skye, in a case where freedom to move was far less and the pressure 

 clearly much greater. 



Mr. UssHER stated that the pillow-lavas of Devon and Cornwall 

 were emitted during the later stages of the eruptions of the 

 Ashprington Series, which in the Totnes area began during the 

 lower stages of the Middle Devonian. Traced westwards, the 

 Middle Devonian eruptions became less frequent and those in the 

 Upper Devonian more important. JN'owhere throughout their whole 

 outcrop had he seen pillow-lavas of so great a thickness as those 

 in the area described by the Authors. The pillow-lavas of the 

 Plymouth area south of Saltash alluded to by Dr. Flett, although 

 well-developed, were not very thick, and were accompanied by tuffs 

 in which he had last Easter found a fragment of slate with Styliola, 

 a fossil characterizing the lower grey beds of the Upper Devonian 

 in the Plymouth and Padstow areas. South of Saltash, near Henn 

 Point, several beds of chert were interbedded with the pillow-lavas, 

 the intercalation forming a small triangular mass in the upper part 

 of the low cliff. In the extension of these spilite-lavas, pillow- 

 structure had very seldom been detected ; but rocks of this character 

 at or about the same horizon were found at intervals, from N'ewton 

 Ahbot westwards to the district described. 



Mr. Clement Reid, replying for both Authors, thanked the 

 Pellows foi the reception of their paper. Dr. Plett had remarked 

 on the slight resemblance between this eruption and that of Mont 

 Pele ; the only resemblance that they suggested was in the steam- 

 cushions on which in both cases the fragments rolled. 



Pillow-lavas were usually associated with fine-grained marine 

 strata, and with deposits such as radiolarian cherts which suggested 

 some depth of water. Perhaps the varying types of pillow might 

 be connected with the different depth of water into which lavas 

 had been ejected, and the consequent variations of pressure and 

 temperature in the boiling water surrounding them. 



