638 J. V. LEWIS — OKIGIK OF PILLOW LAVAS 



Thomson, "of attempting to explain and propound intrinsically untenable 

 views." ^" 



Several observers (Green, 1887; Eeid and Dewey, 1908; Daly, 1914) 

 have relied chiefly on a supposed "spheroidal state" which is assumed to 

 exist at the contact of water with a highly heated lava, and in some cases 

 it is evident that a "spheroidal state" of the lava itself is meant. The 

 Port Isaac (Cornwall) pillow lava is a tremendously vesicular and cav- 

 ernous rock, which Eeid and Dewey conceived to have "rolled on cushions 

 of steam" that emanated in part from the magma and in part was gener- 

 ated from the surrounding water. Thus the whole flow, over 200 feet 

 thick, was believed to have advanced like a liquid, the individual sphe- 

 roids scarcely touching one another until the temperature dropped below 

 the boiling point of the water. The authors assume the individual units 

 were erupted as such, but they do not undertake to explain whether the 

 separation was a part of the process of eruption or was of deeper-seated 

 subterranean origin. Cole and Gregory (1890) thought that such 

 rounded masses had rolled over among themselves deep within an ancient 

 crater and solidified there as pillow lava, subsequent erosion of the upper 

 parts of the volcano having eventually exposed them at the surface. 



Assuming the eruption of vast numbers of such lava balls to be possible, 

 their flowlike movement for a short distance in the manner described, on 

 the sea-bottom or even on the land, is perhaps conceivable ; but this point 

 is of minor importance compared with the question of their origin, which 

 the hypotheses leave unanswered. Geikie (1903) accounted for their 

 production by the "sudden disruption and chilling^^ of the lava on flowing 

 into water, and Eeuning's (1907) theory is very similar. 



Several authors, without appeal to any supposed spheroidal state of the 

 lava (or of the water, which in some mysterious manner induces sphe- 

 roids in the lava), have imagined such highly exceptional physical condi- 

 tions of the flow itself as to render their hypotheses scarcely more prob- 

 able. For example, Gregory (1891), Crosby (1893), Clements (1899, 

 1903), and Van Hise and Leith (1911) have assumed that the lava might 

 be broken up by contraction- jointing while still possessing a very marked 

 degree of liquidity, so that the supposed joint-blocks developed concentric 

 layers and lines of vesicles, a marked flow-structure parallel to their outer 

 surfaces, and a transition from a glassy exterior to a cr}^stalline interior. 

 That such a supposition could be seriously entertained at all bears elo- 

 quent testimony to the "fascinating difficult/^ of the problem. The roll- 

 ing of the angular joint-blocks over one another was supposed to aid in 



122 James Thomson : Seventh Ann. Rept. Belfast Naturalists' Field Club, 1869-1870, 

 p. 28. 



