Hills of which I knew the nature, and which very much resembled those from the banks of the Avon, I had no difficulty in ascertaining that the cement of the latter was composed of magnésian limestone; of which indeed the characters are so strongly marked as to be evident on mere inspection. This breccia consisted of rolled fragments of milk white quartz, and of angular fragments of limestone and sandstone such as are found in the neighbouring inclined strata cemented together by yellow magnesian carbonate of lime; the cement being in great excess. I must refer to Mr. Bright's paper (which I understand will soon be published) for the further description of this rock.
Shortly before seeing the rocks of Hung-road, I had been with the late Smithson Tennant, Esq. to examine the magnesian breccia which he had observed on the Mendip Hills near to the celebrated cliffs of Cheddar. The southern declivity of that limestone chain is there deeply furrowed by wide and extensive combs, in which immense blocks of the breccia many yards in diameter are found lying upon the surface of the limestone. The strata of limestone dip to the south under an alluvial valley, by which they are separated from a low chain of red marl that is found at the distance of about half a mile to the south.
The breccia of the Mendip Hills very much resembles the breccia from the Avon, consisting of fragments of limestone, magnesian limestone, and sandstone cemented together by a yellow magnesia carbonate of lime; but I never discovered in it any of the quartz pebbles which are imbedded in the breccia from the Avon.
Until I had seen the breccia at Hung-road I was unable to account for the presence of these immense insulated blocks upon the sides of the Mendip; but I have since ventured to conjecture that they once formed a subordinate bed in the strata of red marl which are found
