134 Reports and Proceedings — Geological Society of London. 



I. — Geological Society of London. 



l.—Ja?mar?/ 22nd, 1908.— Sir Archibald Geikie, K.C.B., D.C.L., Sc.D., 

 Sec. E..S., President, in the Chair. 

 The following communications were read : — 



1. "The Origin of the Pillow-Lava near Port Isaac in Cornwall." 

 By Clement Reid, F.R.S., F.L.S., F.G.S., and Henry Dewey, F.G.S.^ 



The Upper Devonian strata around Port Isaac consist of marine 

 slates, in which occurs a sheet of pillow-lava over 200 feet in 

 thickness. The pillows measure usually from 2 to 5 feet in diameter, 

 but range up to 8 feet ; masses under 1 foot are rare. The individual 

 pillows are quite disconnected, although moulded on one another and 

 adherent where they touch. Where three pillows approached there 

 "was an angular vacant space, subsequently filled with calcite, which 

 is often altered into chert. Their mutual relations seem to prove 

 that they were soft when deposited, but not sufficiently soft to squeeze 

 into corners. 



Each pillow shows internally a central vacant space or very open 

 sponge, often as much as two feet in length. This is succeeded by 

 a thick shell of exceptionally vesicular lava, which is followed by an 

 outer shell of banded, more or less, vesicular rock. The whole mass 

 is so vesicular that it must have been very light. 



If this lava were subaerial, the lightness would not help us to 

 explain the origin of the isolated pillows ; but the intimate association 

 with fine grained marine strata shows that it was probably submarine. 

 On calculating the proportion of cavity to rock in two of the pillows, 

 the authors find that the specific gravity of the whole mass must 

 have been very low, not greatly exceeding that of sea-water. The 

 lava seems to have been blown out into thick walled bubbles, kept 

 from touching each other by the escaping steam. The whole mass 

 was for a short time in the spheroidal state, and, although composed 

 of a multitude of large plastic spheres, the sheet could flow like 

 a liquid. This eruption seems to have been analogous to that of 

 Mont Pele, described by Dr. Tempest Anderson and Dr. Flett, except 

 that it was submarine instead of subaerial. 



2. " On the Subdivision of the Chalk at Trimmingham (Norfolk)." 

 By Reginald Marr Brydone, F.G.S. 



The object of this communication is to lay before the Societv a 

 sketch-map showing the geographical distribution of the sub-divisions, 

 with a brief accovmt of their distinguishing features. Practically the 

 whole of the Chalk exposed on the foreshore comes under the two 

 main divisions — one composed of («) Sponge -beds, very largely yellow, 

 12 feet, resting on 8 feet of White Chalk; (b) White Chalk without 

 Ostrea lunata, about 9 feet thick ; (<?) White Chalk containing 

 0. lunata, 20 feet ; and the other composed of («) Grey Chalk, about 



^ Communicated by permission of the Director of H.M. Geological Survey. 



