Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/306

212 § 12. The Age of the Lower Brick-earths of the Thames Valley, and of the deposit at Clacton. — In the foregoing Table of distribution of British Postglacial Mammals I have not classified the river- deposits of Clacton, Grays, llford, and Crayford with those undoubtedly Postglacial, because of the conflicting evidence of their faunas as to their true place in the geological scale. The list of Mammalia found in them is inserted in the Table of distribution immediately after those from the forest-bed. We will proceed to sum up the whole of the palaeontological evidence which they offer.

The occurrence at Clacton * of the Rhinoceros leptorhinus of Owen, of Elephas antiquus, Hippopotamus major, Irish Elk, Horse, and Urus may be accounted for equally well by the assumption of its Pre- or Postglacial age; for these animals dwelt in Europe both before and after the Glacial epoch. A new species of Deer, Cervus Browni, is closely allied to the Fallow Deer that ranges in a wild state only over the warm districts around the Mediterranean. The Bison points in the Postglacial direction ; but it will most probably be proved by future investigations on the Continent to have lived in Europe during the Preglacial period. At least the number of Preglacial localities that have been examined is not sufficiently large to give value to the induction that, because it has not been noted, therefore it did not exist. For the most part it has been confounded by naturalists with the Urus and Bos longifrons. The Cave-lion, on the other hand, has been so well determined that the balance of evidence is in favour of its Postglacial age. With its exception, then, there is nothing that forbids the supposition of the Preglacial age of the deposit ; but nevertheless, since the characteristic mammals of the Forest-bed are absent, it would be hazardous to ascribe it to that age. And in the same way, since the Reindeer, Mammoth, Tichorhine Rhinoceros, and other equally common and characteristic Postglacial mammals are also absent, it cannot be said to belong to the class of deposits that contain their remains. We are therefore justified in assuming that it represents in point of geological time an epoch during which some of the more hardy Preglacial and Pliocene species lived under a temperature too severe for the more delicate of their congeners, and not cold enough for the invasion of the Reindeer and the allied Arctic forms.

The Lower Brickearths of the Thames Valley at llford. Grays Thurrock, and Crayford contain the remains of Rhinoceros megarhinus, which has not yet been yielded, in France, Germany, or Italy, by any strata of later age than the Pliocene, and are therefore brought into more intimate relation with that epoch than any other of the deposits undoubtedly Postglacial. But nevertheless the evidence afforded by the Mammoth, Tichorhine Rhinoceros, Cave- lion and Cave-hyaena is in favour of their Postglacial date. And this inference is strongly corroborated by my discovery of a skull of a Musk- sheep at Crayford since the essay on the Lower Brickearths was written†. How, then, can we reconcile the clash of evidence ?


 * Geol. Magazine, vol. v. p. 213.

† Quart. Journ. Geol. Soc. May 1867, p. 91.