164 jir. w. s. boultox ox the [May 1904, 



eighth of an inch across, with little or no interstitial matter except 

 secondary calcite and iron-oxide. The lapilli consist of basalt-glass 

 crowded with felspar-microlites, and in all general characters suggest 

 strongly an analogy with the ' volcanic sand ' of the recent West- 

 Indian eruptions, so graphically described by Dr. T. Anderson & 

 Dr. J. S. Flett. 1 



It is highly probable that this basaltic mass, like other pillowy 

 lavas containing portions of sedimentary material, was ejected under 

 water ; and it is certain, I think, that the tuff or agglomerate was 

 not in the main forced into the air by an eruption and deposited in 

 the sea-water. There is no evidence of sedimentation, or the quiet 

 accumulation of dust and lapilli ; all the appearances point to flow. 

 It might be termed a fluxion-tuff or agglomerate. 



Possibly, if the vent had been situated in very shallow water, or 

 on the land, much of this fragmental material would have been 

 blown into the air, fallen in the water, and settled down quietly on 

 the sea-floor, as, indeed, appears to have been the case with much 

 of the tuff at Middle Hope, 2 miles to the north. At Weston, 

 however, the greater weight of water above may have prevented 

 this, and compelled the fragmental material to flow as lava. Or 

 again, as in the West-Indian examples already cited, the expansive 

 force of the imprisoned vapours may have been sufficient to break 

 up the lava within the vent, but insufficient to do more than just 

 force the tuff over the lip of the vent, whence it flowed along the 

 sea-floor in obedience to gravity, and impelled forward, in part, by 

 the expanding gases. 



One of the most remarkable features of the sheet is the abundance 

 of lumps and irregular masses of limestone, enclosed in the 

 amygdaloidal and ' pillowy ' basalt, or occurring as phacoids and 

 lumps in the tuff}- material. 



In the accompanying diagrams (figs. 5, 0, 7 & 8, pp. 105-67) some 

 of these masses are shown, ranging in size up to 10 or 12 feet, often 

 broken and torn, in part eaten into and absorbed by the basalt, and 

 sometimes tailing off into smaller and smaller fragments. Charac- 

 teristic features are the concave surfaces of the limestone, often due to 

 the fact that the latter occupies an irregular space between a number 

 of spheroids or ' pillows' of the basalt, looking as if either the lime- 

 stone had been absorbed by the hot lava, or, more probably, squeezed 

 into its present shape by the distending and moving spheroidal 

 masses. 



There can be no doubt that this limestone is not secondary, due 

 to the deposition of calcareous material from aqueous solution 

 subsequent to the outflow and consolidation of the lava ; nor has it 

 been deposited as sediment in irregular spaces between the spheroids 

 after the cooling of the basalt ; but it is unquestionabh T part of the 

 calcareous floor upon which the basalt-flow rests. Many of these 

 included masses are oolitic, the structure being visible sometimes to 



: Phil. Trans. Roy. Soc. ser. A, vol. cc (1903) pp. 448-49. 



