Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/763

 From this brief description it is obvious that the term " Forest- bed " is inadequate to express so varied a formation, which, judging by its changes of level and of its fauna, must have continued a very long time ; and I beg to suggest that the " Forest-bed series " would be a better name. The want of proper divisions has led to many complications, especially in the palaeontological department.

It might be supposed that it would be easy to ascertain the position of a bed of such duration and extent ; but such is far from being the case. It has a nearly uniform horizon on the level of the water, and every attempt to reach the bottom of it has been foiled by the water rising.

It is evident that it must lie between the Chalk, or London clay, below and the Chillesford clay above. In the inland section the only two intervening beds are the supposed Mammaliferous crag of Mr. Charlesworth and the Marine crag, which may be seen to advantage at Bramerton and Thorpe. Between these two beds there does not appear to be any break for the intercalation of the Forest-bed series ; for they are deposited in succession upon each other in increasingly deeper water. The Mammaliferous crag was supposed to contain the Mastodon arvernensis, and has therefore been regarded as older than the Forest-bed, which has not been known to yield the Mastodon. Where, then, can the Forest-bed be placed in the inland section, either in point of time or of superposition ?

I beg to suggest the following solution. All the specimens of Mastodon arvernensis, so far as I can ascertain, have been found, together with Elephas meridionalis and several species of Cervus, in a stony bed, one or two feet thick, between the surface of the chalk and the Fluvio-marine and Marine Crags ; and, consequently, those crags, with the exception of a few water- worn fragments and the teeth of Arvicoloe, are nearly non-mammaliferous. It seems probable, therefore, that here may be a break for the intercalation of the Forest-bed, and that the Fluvio-marine and Marine Crags ought to be detached from this stony bed, with which they have hitherto been incorporated under the name of Mammaliferous crag. The stones appear to be derived from the disintegrated chalk, which is worn down both by the chemical and mechanical action of water.

Reference to the accompanying diagram (p. 554) may make this more intelligible. The chalk presents an inclined plane, upon which the beds from the London clay upwards have been deposited in succession. It is evident, therefore, that the chalk (or portions of it) was subaerial until covered by each successive deposit, viz. —

From a, after the Antwerp crag. " b, " Forest-bed. " c, " Freshwater bed. " d, " Fluvio-marine. " e, " " Marine.

Thus there was left a land surface upon the chalk from time to time, on which the mammals of successive periods may have lived. Consequently Mastodon arvernensis might have lived and died upon that

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