310 
ELEMENTS OF GEOLOGY. 
Terehratida sella, Sow. 
Upper Neocomiaii, Hythe. 
Diceras Lonsdalii. Upper Neocomian, Wilts. 
a. The bivalve shell, b. Cast of one of the 
valves enlarged. 
100 miles distant, is truly remarkable. In tbe latter place 
we find no limestone answering to the Kentish Rag’, and the 
entire thickness from the bottom of the Atherfield clay to 
the top of the Keocoraian, instead of being less than 300 feet 
as in Kent, is given by the late Professor E. Forbes as 843 
feet, which he divides into sixty-three strata, forming three 
groups. The uppermost of these consists of ferruginous 
sands, the second of sands and clay, and the third or lowest 
of a brown clay, abounding in fossils. 
Pebbles of quartzose sandstone, jasper, and flinty slate, to¬ 
gether with grains of chlorite and mica, and, as Mr. Godwin- 
Austen has shown, fragments and water-worn fossils of the 
oolitic rocks, speak plainly of the nature of the pre-existing 
formations, by the wearing down of which the Keocomian 
beds were formed. The land, consisting of such rocks, was 
doubtless submerged before the origin of the white chalk, a de¬ 
posit which was formed in a more open sea, and in clearer waters. 
the shells of the Atherfield clay the biggest, and 
most abundant shell is the 
Among 
Fig. 283. 
large Perna 
Mulleti^ of 
which a reduced figure is 
here given (Fig. 283). 
Similarity of Condi- 
tions causing Reappear¬ 
ance of Species. — Some 
species of mollusca and 
otherfossils range through 
the whole series, while 
Perna Mulleti^BQ^h. One-eighth natural size. Others are Confined tO par- 
«. Exterior, h. Part of hinge-line of upper or ticular subdivisions, and 
right valve. Forbes laid down a law 
which has since been found of very general application in 
regard to estimating the chronological relations of consecu- 
