Page:Quarterly Journal of the Geological Society of London, vol. 31.djvu/90

44 44 J. PRESTWICH ON THE QUATERNARY PHENOMENA reddish quartzite derived from the Budleigh-Salterton conglomerates of the New Red Sandstone ; while small pebbles of red porphyries from the Heavitree and Dawlish conglomerates are not rare, and with them are a few pebbles of slate, and micaceous sandstone, possibly from Devonian strata, and one large subangular pebble of a reddish granite, perhaps from Cornwall. With the exception of the few angular fragments of flint and the granite pebble, all the above are well-rolled pebbles, such as might be formed on any exposed shore. The only circumstance indicating ice- action is the presence of the two large subangular blocks of sandstone, not of local origin*, in the eastern section of the beach, and the granite pebble in the western. It is evident that the drift of the shingle was then from the westward and Devonshire coast, and that it was carried to and round the then small Portland headland f. The list of shells from this beach includes many species not before found in any raised beach ; but notwithstanding this the list falls very short of the number of species now found on the Dorsetshire coast. In the raised beach there are 28 species, whereas Mr. Damon gives from this coast a list of 240, of which he informs me that about 40 belong to the littoral zone. With one exception, all the shells are, according to my friend Mr. Jeffreys, of species now living in the British Channel; but I would point out that they are also all species which have a wide northern range — that the Cyamium minutuin was first found in Greenland, where it abounds — and that there is a marked absence of more southern forms. The mass of loam and angular debris overlying the Raised Beach is not only newer than the beach, but is also subsequent to the anti- clinal which raised the north end of Portland ; for at the foot of the present high escarpment originating in that elevation, is a still larger accumulation of this debris. Whether or not this deposit is newer than the other drift-beds with which the raised and denuded Weymouth area is dotted, there is here no direct evidence to prove ; but I take it next in order, in consequence of its connexion with Portland and with the Raised Beach. Similar deposits overlying other raised beaches have often been de- scribed, and have generally been considered to be a " head " or talus of subaerial origin. Mr. Godwin- Austen J, who has very carefully examined into their structure, range, and origin, both on the English stream or floated by river-ice. t It follows from this that it is not necessary to assign a directly distant origin to any of the pebbles of the Chesil Bank, as those from the Devonshire strata are due rather to a transport which took place during the Quaternary period, when the land between the distant headlands on which raised beaches have been found, such as Berry Head, Start Point, Hope's Nose, and Portland, had not been in- dented by the deep bay now sweeping round by Sidmouth, Lyme Eegis, and Bridport. No doubt the coast-line at that period took a straighter course from Hope's Nose to the Bill of Portland. This, however, is a subject which I purpose to discuss elsewhere (lust. Civ. Eng.), as it bears upon questions connected with the present set of the currents and tides, and on the origin of the Fleet. X " On the Superficial Accumulations of the coasts of the English Channel, and the changes they indicate," Quart. Journ. Geol. Soc. vol. vii. p. 118, 1851.
 * If not transported by coast-ice, they may have been driven down by the