E0CKS OF SOMERSET AND DEVON. 369 



We do not know of the occurrence of similar sandy beds on the 

 flanks of the Mendips ; superficial evidence confines them to Broad- 

 field Down. 



With very few exceptions the old Triassic coast line in this 

 area is fringed with conglomerates, sometimes occurring super- 

 ficially as very thin fringes resting on the Palaeozoic rocks, as 

 on Banwell Hill, Sandford Hill, &c, sometimes occupying con- 

 siderable local superficies, as near Wrington, Blagdon, Shipham, 

 Chilcompton, Mells, Croscombe, &c. (in which places they occupy 

 high grounds sloping towards the principal valleys, with minor hills 

 and undulations of their own), sometimes rising in bold hills wrap- 

 ping a nucleus of Mountain Limestone which is here and there un- 

 covered, as at Churchill and Decoy JSTyland, near Cheddar, some- 

 times as inliers in valley bottoms as at Midsomcr Norton, some- 

 times as outliers on the older rocks, as at Slab-Ho (north of Dinder) 

 and near Binegar and jNettlebridge. In all cases where it rests upon 

 the older rocks the conglomerate fills the inequalities in their out- 

 line, presenting a very irregular junction with them in places where 

 the old coast was much indented. 



De la Bcche thus describes the composition of these conglomerates 

 (Mem. Geol. Surv. of Gt. Brit. vol. i. p. 240) :— " The lowest part of 

 the poicilitic, or new red sandstone series in our district has gene- 

 rally been considered as composed of a conglomerate formed of 

 rounded portions or fragments of the subjacent rocks cemented by 

 carbonate of lime, mixed occasionally with so much carbonate of 

 magnesia that the name magnesian conglomerate was applied to it, 

 and magnesian limestone to the limestone into which it sometimes 

 graduates by the disappearance of the pebbles of the subjacent 

 rocks, and the presence of little else than the matter of the cement. 

 Instead of these terms the names of dolomite conglomerate and 

 limestone were suggested by Dr. Buckland and Mr. Conybeare, 

 they at the same time observing that the cement or limestone, in 

 the absence of pebbles and fragments, was sometimes dolomite 

 (carbonate of lime and magnesia) mixed with carbonate of lime, at 

 others merely carbonate of lime. They also pointed out that the 

 pebbles and fragments in tho conglomerate or breccia were those of 

 the nearest hard rock, fragments of coal sandstones being scarce, 

 probably from their texture being commonly too friable to resist the 

 friction to which the other pebbles had been exposed." 



De la Beche (ibid. p. 241), after mentioning the irregular pro- 

 jection of the conglomerates from the margin at intervals into the 

 marl (of which we recall to mind an instance at Dunyate, near 

 Axbridge, on the south side of the Mendip hills), proceeds to say : — 

 " It will be evident that whenever conditions may arise for the 

 deposit of the carbonate of lime, or of carbonate of lime and 

 magnesia, as the case may be, without including the pebbles, the 

 travertino, for such it would be, would cover up the conglomerate 

 beneath. In supposing, therefore, that these dolomitic conglomerates 

 may often have been beaches skirting the land of the time, rising 

 higher and higher up its flanks as such land became depressed 



2d2 



