﻿Vol. 64.] ORIGIN" OP THE PILLOW- LAVA IN CORNWALL. 271 



being ' volcanic bombs, flattened by pressure ; the weathered 

 examples break up into roughly-concentric rings. Many of them 

 are highly vesicular in the middle, and filled with calcite and 

 epidote. The rock is so greatly epidotized as to be green in 

 colour.' On seeing the specimens and photographs now exhibited, 

 and hearing the Authors' lucid description of the structure, 

 he had little hesitation in concluding that the Chilian example 

 belonged to the same class. 



Dr. Tempest Andeeson mentioned two instances of pillow-lava, 

 at Aci Castello in Sicily and Cape Hezkjanos in Iceland, which were 

 both on the sea-shore. A lava-stream which crossed LakeMyvatn, 

 in Northern Iceland, and extended some miles down the valley below 

 the lake, had on its surface a number of spiracles. Some of those 

 in the lake had thrown up cinder-cones 200 or 300 feet high, 

 the ejecting agent being steam furnished by the water in the lake 

 and the mud in its floor. The smaller ones, chiefly on the lava- 

 stream below the lake, where the action was less violent, had thrown 

 out masses of lava comparable in size and shape to pillow-lava, which 

 had formed small cones. As these had been built upon subaerially, 

 there was no coating of mud to separate the masses. If the process 

 had taken place wholly under water, the necessary mud might have 

 been present, and the structure seen at Aci Castello might have been 

 exactly reproduced. 



Mr, G. Barrow drew attention to the similarity in mode of 

 arrangement of the vesicles in these pillow-lavas and those in the 

 broken-open slag-blocks of which the great Tees Breakwater was 

 built. These blocks were obtained from the blast-furnaces, the slag 

 running into small box-shaped iron trucks. In ordinary practice 

 these trucks were emptied so soon as they were filled, and while the 

 bulk of the slag was still molten ; but, when wanted for the break- 

 water, some had to stand for several hours, until a sufficient number 

 of truckfuls had been obtained to make up a train, which was then 

 taken by an engine to the end of the breakwater. In this way, the 

 whole of the slag in some of the trucks became completely solidified. 

 When broken open the blocks proved to be vesicular throughout, 

 the vesicles being arranged roughly parallel to the outer walls of 

 the block, just as the vesicles were parallel to the outer wall of the 

 pillows in the lavas ; and, further, the central part of the slag- 

 blocks was the most coarsely vesicular — in fact, almost hollow. 

 When broken up by the sea, fragments of this slag floated away, 

 some drifting to various parts of the east coast of England, others 

 even to the shores of Holland : they were supposed to be of 

 volcanic origin, and a fair number were brought to the Jermyn- 

 Street Museum, where their true origin was finally determined. 



A point of special importance was the fact that, when the liquid 

 slag flowed into the truck, no explosive gas was given off; the slag 

 flowed very much like water. It was clear that, if gas was present 

 at all, it was occluded. The speaker thought that these gases were 

 set free, if not actually developed, only at the moment of solidifica- 

 tion or crystallization. The slag-bubbles or vesicles were, on the 



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