﻿270 OEIGIX OF THE PILLOW-LAVA IN CORNWALL. [May I908, 



Dr. Fleit remarked that he was under the disadvantage of 

 not having seen the Authors' sections in the field, and there 

 were some respects in which this pillow-lava differed to a certain 

 extent from those with which he was acquainted. It was very 

 highly vesicular and many of the pillows were hollow, and there 

 were strong arguments in support of the hypotheses brought 

 forward. In Anglesey and at Tayvallich hollow pillows occurred, 

 but were few and might be due to weathering. At Saltash and 

 MuUion Island they must be uncommon, for the speaker did not 

 recollect having seen any. The hypothesis contained in the paper 

 rested on facts which might well be believed to be of importance ; 

 but in our ignorance of what went on in submarine eruptions, it 

 was difficult to say how far they could be held to account for the 

 phenomena of pillow-formation. The theory propounded recalled 

 to the speaker's memory the description of a submarine eruption 

 off Pantelleria in 1891, when large bombs were projected and, after 

 rolling on the surface of the water in clouds of steam, finally 

 exploded with a loud noise. These hollow bombs presented some 

 analogy to the hollow pillows of Port Isaac. 



A study of several of the best-known pillow-lavas of Great 

 Britain had led the speaker to the conclusion that these rocks 

 belonged for the most part to a peculiar group — the ' spilites.' 

 Pillow-formation occurred' sometimes in basalts and other rocks, 

 but the best examples were usually spilites. The peculiarities of 

 this group were pointed out by Dr. Teall in describing the spilites 

 of the Southern TJplands of Scotland, namely, that while they were 

 essentially basic rocks, they were rich in oligoclase-felspars. Near 

 Plymouth, some lavas of this type contained phenocrysts of albite. 

 The rocks described by the Authors were of the same character. 



Dr. Teall said that he considered that the communication was 

 an important contribution to our knowledge of pillow-structure. 

 The facts, which were evidently well exposed, had been clearly 

 described and admirably illustrated. When he and Mr. Fox wrote 

 their paper on the rocks of Mullion Island, they were not able to 

 give a satisfactory account of the origin of the structure. The 

 pillows of Mullion Island appeared to agree in form and dimensions 

 with those described by the Authors, but they were much lese 

 vesicular, and he did not think that any theory which postulated a 

 low average specific gravity would apply to that case. That the 

 pillows were individualized at the time of consolidation was certain ; 

 but he had no clear idea as to how they could be produced so 

 uniformly through a great thickness of rock. 



Mr. 0. H. Evans mentioned that the structure described in the 

 paper was well developed in lavas, of probably Mesozoic age, in 

 the Taltal coast-region of the Atacama Desert, in Chile. In the 

 particular instance that he had in mind, the 'pillows ' occurred at a 

 height of 900 feet above the sea, in a stratum of fine tuff, between 

 enormous accumulations of igneous agglomerate forming part of a 

 great formation described by Darwin as of submarine volcanic 

 origin. The speaker had noted them, at the time of observation, as 



