30 LIAS OF ENGLAND AND WALES: 
clay or any other mechanical sediment may be. The conglome- 
rates of Mountain Limestone formed on ancient beaches of the 
Lias are a case in point ; and the pebbles of these conglomerates 
are frequently as well rounded as any we can find on a beach at 
the present day. What has become, then, of the asperities and 
angles which have been worn away during the rounding of the 
pebbles ? The usual reply is, ' Dissolved in the surrounding sea- 
water;' but it seems to me that there is always the alternative 
possibility that it has been carried away as mud in suspension, 
just like any other mud ; and if so, the deposition of such cal- 
careous mud would go far towards explaining the occurrence of 
calcareous septaria and other nodules, which so frequently occur 
in red deposits like those of the Old lied Sandstone and the Triaa, 
as well as in the formation of the muddy Lias limestones of 
Penarth Cliffs, which, with their flat even bedding and numerous 
intercalations of black shales, certainly do not suggest an organic 
origin. A few oysters and such-like shells may have contributed 
to their growth ; but their general appearance certainly is that of 
strata formed from sediment carried in suspension, and very 
different from that presented by a truly organic calcareous rock." :X 
The preservation of conglomeratic limestone-beds in the Lias, 
even where we have evidence of the proximity of old land is rare, 
and equally so is the evidence of transition between such con- 
glomeratic beds and the ordinary limestones. As will be noted in 
the account of the Lias of South Wales, the limestones sometimes 
contain chips of chert derived from the Carboniferous Limestone, 
when no pebbles of that older rock are to be seen. Again on 
the borders of the Mendip Hills, the Lias limestone contains 
pebbles of quartz derived from the Old lied Conglomerate, while 
pebbles of Carboniferous Limestone may not always be found. 
Of the limestone-conglomerate, some of the best examples are met 
with at Vallis, near Frome, and at Southerndown, near Bridgend. 
It seems to me that the Gotham or Landscape Marble that 
occurs at the base of the White Lias (RhaBtic Beds) affords 
evidence both of sedimentary deposition and of subsequent con- 
cretionary action. In the more persistent layers of rock we find 
only compact and banded limestone with no arborescent markings. 
Thus the dark argillaceous sediments that prevailed previously, 
when the Black Avicula-contorta Shales were laid down, had some 
later influence amid the purer calcareous mud that prevailed 
during the formation of the White Lias. Where the arborescent 
limestone occurs, it is in the form of impersistent masses, and 
these are characterized by a crinkly surface that appears due to 
contraction, because thin films that correspond to the irregularities 
of the surface, may often be flaked off. I have suggested 
that the arborescent markings were produced by the irregular 
admixture of layers of dark mud amid the lighter calcareous mud 
during the consolidation and shrinking of the sediments ; the 
shrinking as a rule only affecting the upper portions of the layer.f 
* See also De la Beche, Mem. Gei.l. Survey, vol. i. pp. 2>84, 293. 
f <ieol. Ma^. 1892, p'. 110. 
