192 
ELEMENTS OE GEOLOGY. 
does not imply a temperature higher than that now prevail¬ 
ing in the British Isles. There must have been a subsid¬ 
ence of the forest to the amount of 400 or 500 feet, and a re¬ 
elevation of the same to an equal extent in order to allow 
the ancient surface of the chalk or covering of soil, on wdiich 
the forest grew, to be first covered with several hundred feet 
of drift, and then upheaved so that the trees should reach 
their present level. Although the relative antiquity of the 
forest-bed to the overlying glacial till is clear, there is some 
difference of opinion as to its relation to the crag presently 
to be described. 
Chillesford and Aldeby Beds.—It is in the counties of Nor¬ 
folk, Suffolk, and Essex, that we obtain our most valuable 
information respecting the British Pliocene strata, whether 
newer or older. They have obtained in those .counties the 
provincial name of “ Crag,” applied particularly to masses 
of shelly sand which have long been used in agriculture to 
fertilize soils deficient in calcareous matter. At Chillesford, 
between Woodbridge and Aldborough in Suffolk, and Alde¬ 
by, near Beccles, in the same county, there occur stratified 
deposits, apparently older than any of the preceding drifts 
of Yorkshire, Norfolk, and Suffolk. They are composed at 
Chillesford of yellow sands and clays, with much mica, form¬ 
ing horizontal beds about twenty feet thick. Messrs. Prest- 
wich and Searles Wood, senior, who first described these 
beds, point out that the shells indicate on the whole a cold¬ 
er climate than the Red Crag; two-thirds of them being 
characteristic of high latitudes. Among these are Gardium 
Grcenlandiciim^ Leda limatula^ Tritonium carhiatum^ and 
Scalaria Grcenlandica, In the upper part of the laminated 
clays a skeleton of a whale was found associated with casts 
of the characteristic shells, Nucula Gohholdim and Tellina 
ohliqua^ already referred to as no longer inhabiting our seas, 
and as being extinct varieties if not species. 
The same shells occur in a perfect state in the 
lower part of the formation. Natica helicoides 
(Fig. 117) is an example of a species formerly 
known only as fossil, but which has now been 
found living in our seas. 
At Aldeby, where beds occur decidedly simi¬ 
lar in mineral character as well as fossil re¬ 
mains, Messrs. Crowfoot and Dowson have now 
obtained sixty-six species of mollusca, comprising the Chilles¬ 
ford species and some others. Of these about nine-tenths 
are recent. They are in a perfect state, clearly indicating a 
cold climate; as two-thirds of them are now met with in arc- 
Fig. IIT. 
Natica helicoides^ 
Johnson. 
