40 



GEOLOGY OF THE BRISTOL COALFIELD. 



together with Dolomite. Thus at CHfton and Durdham downs, 

 s made up of Carboniferous limestone. On the flanks of the 

 Mendip and Quantock hills, pieces of Old Red Sandstone occur, 

 and on Brandon hill, blocks and pebbles of Millstone grit. 



The average thickness of this conglomerate is 20 to 30 feet, 

 although in a few spots as at Holcombe it exceeds 80 feet. Like 

 all such deposits, the largest and heaviest blocks went to the base 

 till they rested on the upturned edges of the limestone or sand- 

 stone, to be covered up in their turn by the smaller pebbles and 

 sand, just as we now find to be the case on the shores of Cornwall 

 and Wales. A characteristic example may be seen in the the 

 section of an ancient beach on the western edge of Clifton Down. 



Fig. 21. — Ancient Sea Beach, 



a — ^Dolomite, b — Dolomitic breccia, c — Large blocks cemented with 

 Dolomite, d — Carboniferous Limestone. 



On examination of the broken pieces their age is proved 

 by bits of Coal, Spirifers, Corals, &c. 



For some time this conglomerate was supposed to be of 

 Pe rmian age, because it resembled in many respects the rocks o 

 Perm in Russia, and to a large extent developed in a portion of the 

 North of England. Oar lamented member W. Sanders, Esq., 

 F.R.S., first demonstrated it to be the base of the Keuper. 



It has been omitted in his large map, because he did not 



