Vol.58.] 10 THE PHYSICAL GEOGRAPHY OF THEIR AGE. 193. 
VIII. Tux Dotomiric ConGLOMERATE. 
In former years I often saw, but without minutely studying, 
sections of this deposit, which has been so admirably described by 
Mr. R. Etheridge.’ The following is a brief summary, chielly 
derived from his paper, of facts which are germane to this investi- 
gation. The Dolomitic Conglomerate fringes the Mendip Hills 
and other prominences of Carboniferous Limestone, almost sur- 
rounding the Somerset and Gloucestershire Coalfield; it occurs 
over an area probably about 140 square miles in extent, and 
at various elevations up to about 300 feet above sea-level, rarely 
exceeding 30 teet in thickness. It can often be traced horizontally 
into a sandstone, is interstratified with the Keuper Marl, and 
sometimes two beds may be seen in an open section, separated one. 
from another by 3 or 4 feet of the latter: the breccia itself 
occasionally exhibiting signs of stratification. The fragments vary 
in size, but usually are less than 3 inches in diameter, though 
much larger blocks—a yard or so in diameter—sometimes occur. 
They generally are either angular, or only slightly worn at the 
edges, but occasionally are well enough rounded to make the name 
‘conglomerate’ the more appropriate. The great majority are 
of Carboniferous Limestone, though Millstone Grit, Carboniferous 
Sandstone, and even Old Red Sandstone are sometimes found: the 
materials depending on the character of the adjacent rock-masses.* 
These are embedded in a reddish marly or sandy matrix, more or 
less cemented by calcite or dolomite, so that the breccia sometimes 
makes an excellent and a very handsome building-stone. It was 
regarded by Sir Andrew Ramsay,* with whom Mr. Etheridge practi- 
cally agrees, as a shore-deposit, composed of materials which had 
mainly been formed on old land-surfaces, but had been more or less 
worked up, as these subsided, by the waters of the Keuper lake or 
inland sea. It may be sometimes cliff-talus, but even in Upper 
Triassic times the land-surface must, I think, have been a region ot 
craggy downs, rather than of great hills with precipitous flanks. 
If so, the fragments must have mainly accumulated on the bare 
limestone-slopes, from which they were either swept down more or 
less tumultuously into the water, or else washed off and re-arranged 
as shingle-beaches during a period of slow subsidence. In an 
inland sea the fragments, though most of these are of rock no 
harder than Carboniferous Limestone, would not generally be much 
rounded ; for there tidal action would be inappreciable, and that of 
waves seldom strong. Striations have been observed, though not 
often, on the surfaces, but these probably have been produced 
subsequently by earth-movements.* 
1 Quart. Journ. Geol. Soc. vol. xxvi (1870) p. 174. 
2 In the Quantock district (for instance) breccias, referred to the same age 
by Mr. Etheridge, consist of Devonian rocks. 
* Quart. Journ. Geol. Sve. vol. xxvi (1870) p. 191. 
* It may be well to mention that a breccia occurs at the base of the Bunter 
in Southern Shropshire and Northern Worcestershire. It reaches a maximum 
of 60 feetor so near Kidderminster, but is not important enough to call for 
special notice. Prof. Hull thinks that the fragments have not travelled far; see 
Mem. Geol. Sury. 1869 ‘ Triassic & Permian Rocks of the Midland Counties ’ 
p. 44. 
