﻿Yol. 64.] OEIGIN OF THE PILLOW-LAVA IN CORNWALL. 269 



with that described by Dr. J. S. Plett & Dr. Tempest Anderson,^ in 

 the West Indies, except that, instead of being subaerial, it occurred 

 at the bottom of the sea. The particles which gave off the steam 

 were not small grains and pebbles, but were masses of 2 or 3 feet 

 or more in length, and having nearly the same specific gravity as 

 sea-water, they rolled as easily as the smaller particles in air. The 

 whole sheet, though composed of large nearly-solid spheres, must 

 have moved almost like a liquid. This reduction of weight caused 

 by the buoyancy of the vesicles would be still further increased, if 

 we allow for the jacketing of the spheroids for a time by a shell 

 of escaping steam. 



It seems, therefore, that the lava was in a true spheroidal state, 

 each large drop ejected swelling up independently and forming a 

 pillow more or less surrounded by escaping steam, so that the 

 flowing mass on the sea-bed formed a mobile sheet of rolling 

 spheres, seldom touching one another till they cooled. As soon as 

 the steam condensed, however, water would be sucked into the 

 vesicles, and the pillows would settle down. 



EXPLANATION OF PLATES XXVII & XXVIII. 



Plate XXVIl. 



Photograph of part of the cliff-face at Pentire, in St. Minver, showing 

 several contiguous pillows in a thick mass of lava. In the centre is shown, 

 between the pillows, a cavity afterwards filled with calcite. The pillows are 

 partly moulded on each other, but have not been squeezed into narrow spaces. 

 Most of the sections in this photograph are tangential, and do not intersect 

 central cavities ; but one pillow above, and another below, the hammer show 

 this hollow, out of which the calcite has been weathered. 



Plate XXVIII. 



This plate illustrates the internal structure of a single pillow from the same 

 locality. The section probably is nearly central, and shows the central cavity 

 surrounded by a spongy zone, outside which is seen banded, more or less 

 vesicular rock. 



DiSCFSSION, 



The Pkesident remarked that all the examples of pillow-lavas 

 with which he was acquainted were undoubted^ true lavas, and be- 

 longed to submarine eruptions. Some of them, however, must have 

 been poured out in shallow water, as was particularly observable 

 in the case of the Lower Carboniferous basalts of the Fife coast. 

 These lavas were thin sheets, often not more than 15 or 20 feet 

 in thickness, and they, as well as the associated tuffs, were inter- 

 calated among shallow-water deposits, such as cyprid-shales and 

 limestones, coal-seams with fire-clays, thin sandstones and iron- 

 stones. Some of the basalts had caught up portions of the mud 

 on the sea-bottom, but in others the muddy, sandy, or ashy sedi- 

 ment of the next deposit had fallen into the interspaces between 

 the pillows. 



1 Phil. Trans. Roy. Soc. ser. A, vol. cc (1903) pp. 353-553. 



