Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/161

1869.] DAVIDSON—PEBBLE-BED BRACHIOPODA. 71 and others have been zealously at work collecting additional information, and every specimen that might assist in determining the age of the rock from which these drifted pebbles were derived has been carefully preserved*.

The Brachiopoda were transmitted to me at various times for examination. I have studied and compared them, so far as this was compatible with the preservation of the specimens. These fossils occur in the shape of internal casts and external impressions only; but by the aid of gutta-percha it was often possible to reproduce the external as well as the internal surface of both valves, and thus describe and figure the entire shell in as complete a manner as if taken from the sea. There are, however, a number of species which have occurred but once, or very rarely, and in so incomplete a state of preservation that it has frequently been impossible to arrive at a decided or satisfactory specific identification; and in such a difficult problem as the age of these pebbles at Budleigh Salterton, one cannot be too cautious in the identification of the fossils which they contain.

After the careful examination of several hundred specimens, I have been able to recognize from thirty-seven to forty species of Brachiopoda, besides many fossils referable to other classes. Of some of the former, descriptions have already been published in my Monograph of Silurian Brachiopoda.

The rock or pebbles containing these fossils, as described by Mr. Vicary, "is generally a Sandstone, but sometimes so compact as to assume a quartzite character." The colour of the rock is very variable, being either white, grey, yellow, or more or less strongly tinted with red. The size of the boulders varies considerably; they have been completely rounded and polished by the action of the waves over a rough bottom; and a comparatively small number seem to contain fossils. The pebble-bed, on the coast, in its thickest part, is not much more than 100 feet thick; it caps a ridge of hills running inland to the north full twelve miles, extending down both sides of these hills, in some places, to the extent of three or four miles. Most of the fossils are found near the coast; and I am informed that some few have been likewise met with in the Straightway pits, where pebbles are broken for the roads; but none were ever found in that locality by Mr. Vicary. The pebbles are largely distributed with flints in the gravel-beds of the neighbourhood†.

From the appearance of the pebbles and their accumulation in the cliff at Budleigh-Salterton and in other spots, irrespectively of their fossil contents, it would be natural to arrive at the conclusion that they all belong to a rock of the same age, transported thither at a period subsequent to its formation; and this view is, rightly or wrongly, entertained by several geologists. It has been likewise asked:—How can fragments of rock, varying in age, be accumulated in the same pebble-bed? They could not have come from different localities; and if


 * The discovery of by far the largest number of species is due to Mr. Vicary.

† See also a paper, by Mr. Pengelly, "On the Denudation of Rocks in Devonshire."