LOWER LIAS : HARPTREE. 125 
Encrinites occur in some places, as at Emborrow, but these are 
no doubt derived from the Carboniferous Limestone. 
The beds below the chert were well exposed near the cottage 
at Harptree Hill, in a pit showing hardened reddish-brown 
micaceous sand with Pallastra arenicola. In this neighbourhood 
I obtained also Avicula contorta and Pecten valoniensis, thus 
proving the Hhastic age of the lower beds.* 
The precise junction of Rhaetic Beds and Lower Lias is 
not now to be determined, but in the section published by 
Weaver, he noted a bed of siliceous breccia at the base of the 
cherty beds and overlying the sands. This breccia is said to con- 
tain fragments of sandstone (probably Old Red Sandstone), quartz, 
&c.f Whether this should be taken as the approximate boundary 
of lihjetic Beds and Lower Lias, or whether beds of the age of 
the White Lias are included in the overlying cherty beds, cannot 
be decided from the evidence before us. All we can say is that 
the fossils obtained from the cherty beds represent the zone of 
Ammonites planorbis ; and it is noteworthy that they include forms 
characteristic of the Sutton Stone of Glamorganshire. (See p. 112.) 
Dr. G. J. Hinde, who has examined the rocks in situ, informed me (October 
1887) that the chert " seems everywhere to have the same porous structure, and 
the silica is in peculiar crystalline grains, resembling frosted sugar. The cavi- 
ties seem, in part at least, to be due to the dissolution and removal of 
fragments of calcareous organisms ; amongst them [ can recognise the casts of 
fluted Echinoderm spines. The silica appears to have infilled the minute inter- 
spaces between the calcareous fragments to form a matrix, and subsequently 
they were dissolved out. After several hours hammering, both at the pit and 
at the material on the fields, I only found one fragment in which sponge- 
spicules were present, and this was by the road-side between the Castle of 
Comfort and the East Harptree pit. I had at first some doubt whether the piece 
might belong to the Liassic Chert, but I now think it does, for I can find casts 
of the minute Echinoderm spines, and though the larger part is compact there 
is a porous portion of the same character as the rest of the Lias Chert. This 
solitary specimen gives some grounds for the belief that if we could find the 
Lias Chert un weathered, it would show its derivation from sponge-remains, the 
same as the Carboniferous Chert. * * * Nevertheless 1 do not think 
it would be right to conclude that the Liassic Chert has been derived directly 
from the sea-water, though there is but scant evidence of its organic origin ; 
for it is quite possible that its present granular crystalline condition may have 
resulted from the alteration of the organic silica of Sponge-spicules or other 
siliceous structures. There is a somewhat similar condition of the silica in the 
Carboniferous (so-called) chert at Bakewellin Derbyshire. * * * I found 
plentiful traces of spicules in blocks of Carboniferous Chert in the roadside- 
walls between Wells and the Mendips ; and curiously enough two fragments 
of cherty rock, which you had given to Mr. A. Gillett, from the Mendip rocks, 
showed spicules very distinctly, and it is therefore probable that the Mendip 
Chert is of the same character as that in the Carboniferous rocks of Yorkshire." 
In attempting to account for the origin of this Cherty Lias 
(1868-71), I was impressed with the fact that, around Green 
Down Cottage, the Lower Lias and underlying ^Vhite Lias 
present their ordinary characters. The Lower Lias there consists 
of hard argillaceous limestones and clays with Lima (/igantea, and 
* See also Tawney, Quart. Journ. Geol. Soc., vol. xxii. p. 79. 
f Some of the Druidtcal stones of Stauton Drew are blocks that resemble this 
and other beds belonging to this cherty series. 
