﻿268 ME. C. HEID Ai!JD MB. H. DEWEY ON THE [May I908, 



The exceptionally-spongy character of this lava is very striking, 

 and light was thrown on it from an unexpected quarter. Happening 

 to look at an article in 'Science Progress' for October 1907, on 

 ' Bread-making '* by Mr. A. E. Humphries, the ex-President of the 

 !N^ational Association of British & Irish Millers, we were struck by 

 the statement as to the bread-making qualities of different flours. 

 Chemical composition seemed to throw little light on this quality; 

 it depends on the physical capacity of the dough for retaining the 

 bubbles of gas set free by the yeast. Wheat-flour has this capacity, 

 barley and rye-flours do not possess it ; and wheat-flours vary con- 

 siderably in this respect, though their chemical composition varies 

 so little. Applying this experience to the lava, it would seem that 

 this molten mass was not only full of steam, but was in a state to 

 retain great part of the steam in the vesicles. We will not venture 

 to say exactly why this particular lava will blow into bubbles, and 

 another will not; but in this case we have a lava which, notwith- 

 standing its great and probably sudden expansion, did not blow to 

 pieces or lose most of its gas, but retained enough of its steam 

 to swell up and form a sponge. 



The lightness of the pillows above described does not much 

 assist us, if the eruption were subaerial. The lava, however, 

 occurs in the midst of purely-marine strata, and in all probability 

 represents a submarine eruption, as Dr. Teall has suggested is the 

 case with other pillow-lavas. When looked at from the point of 

 view of submarine eruption, the highly-vesicular character of the 

 rock assumes extreme importance. The space occupied by vesicles 

 is so great as to form a very large percentage of the total cubic 

 contents ; and, when we take into account also the big central 

 cavit}^ the question arises whether some of the pillows would not 

 be so light as actually to float in sea-water, so long as the vesicles 

 were filled with steam. 



If •some of the spheroids were so buoyant as to float, some of 

 these more buoyant masses may be represented by the puzzling 

 isolated pillows, which look so like bombs, but seem never to 

 have been broken by a fail. Even in the main mass we have been 

 unable definitely to find broken spheroids ; the angular clinkers 

 which occur in the quarry west of Trelights may be such, but the 

 better sections in the cliff do not appear to show any. 



Yery little tuff is associated with this particular eruption, although 

 some of the inland localities seem to show fragmental material 

 partly replacing the sheet of lava towards its thin edge. Unfortu- 

 nately, however, where the sheet becomes thin it has been sheared 

 to pieces, and therefore we now feel convinced that certain fragmental 

 rocks at first thought to be tuff are merely sheared lava. A good 

 section showing this passage of vesicular lava into sheared frag- 

 mental rock, may be seen in the northern bank of the Camel, about 

 a mile north-west of Wadebridge. The top and bottom of the 

 thick sheet also commonly show fragmental rock resembling tuff. 



Thus we appear to be dealing with a type of eruption comparable 



