126 LTAS OF ENGLAND AND WALES: 
at the base there is a thin laminated sandy limestone with Ostrea 
liassica in profusion. South of the cross-roads at Egar Hill, Lias 
limestone occurs, tough and compact, and not unlike Carboniferous 
Limestone, n.s are certain beds of the Lias near Shepton Mallet 
and in South Wales. Ammonites Johnstoni was here met with. 
The Rhtetic Sands, before mentioned, occur over the ordinary grey 
marls, which pass downwards into the red marls of the Keuper. 
The alterations therefore are local and abrupt, and it is impor- 
tant to notice that we find these siliceous modifications in both 
Lias and Trias. The sandy modifications of the Rhretis Beds, 
above the Grey Marls, are significant, and point to marginal 
deposits, derived probably from Old Red Sandstone ; but we have 
no evidence to prove that the Lower Lias was here represented 
by sandy sediments. In any case the beds have undergone altera- 
tion. I compared this Cherty Lias with that at Portrush in the 
N.E. of Ireland, where the argillaceous beds have been metamor- 
phosed into a dark lydian stone by contact with basalt. On the 
Mendips we have no evidence of eruptive rocks nearer than 
Downhead. It is, however, possible there may have been some 
intrusive rock which did not find its way to the surface, but made 
its influence felt by the agency of heated water. This, in short, 
is the hypothesis I ventured to advocate ; and the fact that the 
Triassic Beds in the neighbourhood are also of a peculiar cherty 
nature, lent some support to the notion.* I may add that near 
Frome the Inferior Oolite, in places, exhibits a very cherty 
character. De la Beche has called attention to this,f and I shall 
have occasion to refer to the subject in a subsequent volume. 
Radstock and Paulton. 
The Lias in the area north of the Mendip Hills around Rad- 
stock, Paulton, and Timsbury may be divided as follows : 
FT. IN. FT. IN. 
T . /Blue clays - - - -100 to 120 
t Limestone and clays - - - 7 6 24 
Rhsetic Beds. White Lias (the top layer known as the Sun Bed). 
The Inferior _Oolite in this tract rests directly on the Blue 
Clays, and there has been some difficulty in fixing their precise 
age. Do they represent Upper, Middle, and Lower Lias, or 
portions only of one or more of these formations ? The difficulties 
are mainly due to the rarity of sections, the want of palasonto- 
logical evidence, and the apparent absence, between Bath and the 
Mendip Hills, of any prominent beds of Marlstone. 
Palaeontological evidence, however, shows that in this region 
the zone of A mmoniies Jamesoni, and locally that of A. Henleyi, 
are represented, like the lower zones, by one or more bands of 
limestone ; so that as regards the main mass of the Lower Lias, 
the calcareous beds prevail over the argillaceous. Therefore it 
* Geol. Mag., 1871, p. 400. 
f Mem. Geol. Survey, vol. i. p. 287. 
