﻿266 WR. C. EEID AND MR. H. DEWEY ON THE [MaV 1908^ 



rock. The lava is highh' vesicular, and, hoTrever thick the mass 

 may be, it usually consists of an accumulation of detached pillows 

 piled one upon another. The pillows seem to have rolled over and 

 over, and to have settled down while still sufficiently soft to take 

 impressions of the contiguous spheroids. They cannot, however, 

 have been very fluid, for triangular spaces were commonly left where 

 adjacent spheroids did not fit. These interstices and the vesicles 

 were afterwards filled with nearly pure calcite, which in some 

 places has been replaced by chert. 



Not only were spheroidal pillows of lava piled up to make a 

 single sheet, but in a few cases detached spheroids can be found 

 isolated in the shales, a few inches, or even a few yards away from 

 the main mass. It is unfortunate that, although the main mass may 

 have completely resisted shearing (as is shown in the photographs. 

 Pis. XXYIl & XXTIII), yet there has been considerable movement 

 at the top and bottom of the sheet. This usually makes the exact 

 relation of the lava to the sediments somewhat difficult to trace out. 

 "We can only say that there seems occasionally to be a little alteration 

 of the sedimentary strata, and also a small amount of disturbance 

 and inclusion of sedimentary material (apparently mud, not slate) 

 near the base of the lava. The greater number of the supposed 

 inclusions have, however, a totally-different origin, for they are 

 merely infiltrations of calcite. 



The shearing at the junction of the lavas and sediments usually 

 makes it difficult to determine whether detached pillows owe their 

 isolation to this cause, or were detached from the main mass, at 

 the time of their emission, while still hot. In some cases there 

 seems no doubt that detached spheroids showing in section a 

 perfectly-rounded outline, with no impression of other spheroids, 

 are in their original position. The difficulty of photographing a 

 cliff-face, only accessible by climbing, has made it impossible to 

 obtain a satisfactory illustration of one of these occurrences. 



As already mentioned, the main mass of lava seems to be of great 

 thickness, and apparently it is the accumulation of a single out- 

 burst, during which the rock flowed only a few miles. "We have to 

 explain, however, in what manner a sheet of this sort can be built 

 np of completely-detached spheroids and be highly vesicular from 

 top to bottom. The structure is quite unlike that of ropy lavas, 

 or of any recent lavas of which we have heard. If individual 

 spheroids be studied, we think that the clue may be obtained. 



The pillows of which the mass is made up vary in size from about 

 1 to 8 feet in diameter, the majority measuring perhaps 2 or 3 feet ; 

 pillows having a diameter of less than a foot are rare. Each 

 spheroid has definite boundaries ; and, even where the surfaces are 

 in close contact, they are separated by a thin skin from the adjacent 

 spheroids. As already mentioned, though somewhat flattened and 

 dented, the pillows were not soft enough to mould themselves to 

 the underlying pillows at all points, and triangular and more or 

 less rectangular spaces occur where the pillows do not meet. 



The structure of the spheroids is shown in the numerous natural 



