CARBONIFEROUS VOLCANIC ROCKS OF SOMERSET 205 
and green matrix, and containing lenticular ashes of vesi- 
cular lava, phacoids (often broken and torn) of lime- 
stone, and higher up the cliff larger spheroidal lumps of 
lava. The whole band suggests forcibly the augen-struc- 
ture characteristic of gneisses. It probably represents, 
however, a torrent of agglomeratic material that flowed 
down a slope on the surface of an already-extruded bed of 
lava, carrying in among the finer lapilli larger, irregular, 
and plastic masses of scoriaceous basalt-lava of the nature 
of bombs, together with lumps and fragments of lime- 
stone, which from their form and broken character suggest 
that they were ejected from the vent with the basaltic 
material. In all cases where the phacoidal or lenticular 
structure is seen, whether on a large or on a small 
scale, the material forming the ground-mass is frag- 
mental and tuff- like, while the included phacoidal masses 
consists of vesicular lava or limestone, or very occasionally 
masses of coarse tuff. A thin slice of the typical tuffy 
matrix shows small sub-angular or rounded, closely-fitting, 
equal-sized lapilli, about an eighth of an inch across, with 
little or no interstitial matter, except secondary calcite 
and iron-oxide. The lapilli consists of basalt-glass crowded 
with felspar microlites, and in all general characters sug- 
gest strongly an analogy with the ‘ volcanic sand ’ of the 
recent West Indian eruptions, as graphically described by 
Dr. T. Anderson and Dr. J. S. Flett.^ 
“ It is highly probable that this basaltic mass, like other 
pillowy lavas containing portions of sedimentary materials, 
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. 
1 Phil, Trans. Roy. Soc., ser. A., vol. cc. (1903), pp. 448-49. 
