168 mr. w. s. boulton ox the [May 1904, 



the rest being filled with, pure white, secondary calcite, and in some 

 cases quartz ; indeed, in a few instances, rounded or oval bodies up 

 to a foot in length consist entirely of silica. These may be lumps 

 of limestone, burnt and hardened on the outside by the hot lava* 

 their centres being subsequently removed in solution, the hollows 

 thus formed serving as receptacles for secondary calcite or quartz ; 

 while in a few cases the whole lump of limestone has been replaced 

 by silica. It is possible, however, that some of them may be large 

 vesicles rilled with secondary minerals. 



But, in most cases, the general shape and behaviour of the lime- 

 stone-masses, particularly between the spheroids of basalt, seem 

 rather to suggest that the calcareous material must have been only 

 in part consolidated, so that it behaved as a pulverulent or more or 

 less plastic substance, and got roiled in or picked up by the lava,, 

 and was able to fit itself in between the moving and distending 

 spheroidal masses. 



In this connection, it is interesting to compare the general shape 

 and appearance of these included masses with those in other 

 localities, as, for example, in the Arenig lavas of Ballantrae and 

 elsewhere, with their marked pillowy structure, so well illustrated 

 and described by Sir Archibald Geikie and the officers of the Geolo- 

 gical Survey. 1 There the included material is jasper, radiolarian 

 chert, graptolite-shale, and limestone. In the memoir describing 

 these rocks in the Ballantrae district it is stated (op. cit. p. 432) 

 that 



' the calcareous matter does not seem to have penetrated far down through 

 the successive beds, being confined mainly to the surfaces of the flows. 



In the case at Weston it must be admitted, as already pointed out, 



that the calcareous material did not come from above, 



but from the underlying floor. 



In the account of these Weston rocks by the officers of the Geolo- 

 gical Survey, it is suggested that the vent from which the rocks of 

 Spring Cove were derived lay somewhere to the west, where now 

 the Bristol Channel lies ; but from the fact that the included masses 

 of limestone dwindle rapidly in size and number from north to south, 

 and that the lenticular sheets of lava and tuff, representing indi- 

 vidual minor flows, also slope from north to south, it would seem 

 more probable that the vent lay somewhere to the north of this 

 Spring-Cove exposure. 



Except for the presence of lapilli of basalt in the base of the 

 limestone resting at once on the basalt, it might be difficult to show- 

 that the whole is not an intrusive sheet. The conditions in these 

 submarine flows appear to be very like those in a sill or intrusive 

 sheet, where, as Prof. Lapworth has suggested, we may get tuffs, 

 lava, and included masses of sedimentary material confusedly mixed, 

 and drawn out into lentieles as here described. 



1 Mem. Geol. Surv. (1890) 'Silurian Rocks of Britain' vol. i, Scotland. 



