198 FLUVIO-MAEINE DEPOSIT ON THE COAST OF ESSEX. 



trees, but the successive accumulations of peaty matter, and 

 their various thicknesses, which are seen alternating with 

 layers of marine and fresh-water shells, mingled together in 

 the same bed, as shown in the section, (fig. 9). 



Moreover, this thick mass of peat affords us sufficient evi- 

 dence that the marine shells were introduced to the different 

 beds by various irruptions of the sea into this fresh-water 

 lake, while the beds of peat were there forming. And it is 

 worth remarking, at the same time, that there does not ap- 

 pear to be any admixture of marine fossil shells in the lower 

 stratum (No. 7.) of the section, in which such a great number 

 of bones of Mammalia have been found associated with P«/w- 

 dina, Valvata, Lymnea, Planorbis, Ancylus, Helta:, Vertigo, 

 Clausula, Unio, Cyclas, Cypris, and seeds of Chara ; the 

 last being very numerous. 



The lowest stratum appears to have been formed under dif- 

 ferent conditions to those which prevailed when the beds of 

 mixed shells above it were deposited. 



The fluviatile shells of the upper beds were doubtless de- 

 rived from this lowest stratum, as the fluviatile species of the 

 upper beds occur in great abundance in the lowest, namely, 

 No. 1 of the section. But at the same time there are mol- 

 luscous genera in the lowest stratum, which have not been 

 met with in the newer beds : for instance, the Unio, which 

 occurs in great plenty in the lowest bed, has not been found 

 in the upper beds. 



If this is the same species of Unio as that which is found 

 fossil at Grays, and is figured at page 548, vol. ii. n. s. of 

 this Magazine, according to the description there given, it 

 does not appear either to be common as a fossil, or to be 

 known as a recent British species. 



Geologists have termed these fossil mammalian remains, di- 

 luvial, from the circumstance of their being very frequently 

 found in gravel; but oftentimes their high state of preserva- 

 tion is opposed to the idea of their being drifted from any 

 great distance, in company with the rough and hard mate- 

 rials of which gravel is generally composed. And from what 

 I have observed of the bones of the larger Mammalia which 

 were deposited in the lower bed of this formation, the good 

 condition of these and of all the fossil bones that have hitherto 

 been found along this coast, leads me to infer that their histo- 

 ry is more closely connected with the fresh-water beds, than 

 with the gravel. 



I have had opportunities of observing the relation between 

 fossil Mammalia, and the lacustrine beds in which they were 

 found, at other places as well as on this coast, viz. at Stutton, 



