﻿1861.] 



PEESTWICH BOULDER-CLAY. 



451 



estuary and river were inhabited by the Bissoce, Lacuna vincta, the 

 variety of Cardium, Tellina solidula, the Gyrena (or Corbicula), 

 and probably the Ostrea ; the shells being carried down into the sea 

 by the river. 



" 2nd. Below the beach, referred to underneath, a belt of loose 

 stones of different sizes, with patches of mud ; and here lived the 

 Purpura, Mytilus, Nassa, Littorinm, and Balanus. 



" 3rd. An exposed patch of chalk, which produced the Pholas. 



" 4th. A wide and deep patch of sand with gravelly mud, inha- 

 bited by the typical Cardium, the Mactrce, and the Mya. Only un- 

 covered at low spring-tides. 



" 5th. Seaward, at a depth of from 20 to 30 fathoms, and at a 

 distance of not more than two or three miles from the shore, where 

 the ground was sandy, we might expect to find the Buccinum, Fusi, 

 Naticai, Corbula, Nucida, Astarte, Yenus, Dentalium, Lacuna crassior, 

 variety of Mactra subtruncata, the Tropha, and Mangelim. All 

 these were within the range of the tidal current, and may have 

 been thrown up on a pebbly beach, which would account for their 

 generally broken and rolled condition." 



In addition to the above I may notice the occurrence of a few 

 small fish vertebras and of annelid borings in the pieces of chalk. I 

 was informed that some large bones* had been found two or three 

 years since ; but the only trace of bone I found was an undetermi- 

 nable fragment reduced to a pebble-form. 



As it would appear that none of the shells are in their original habitat, 

 it becomes a question whether they are all contemporaneous, and of the 

 same date as the gravel in which they are imbedded. With one or 

 two exceptions I am inclined to answer in the affirmative. The shells 

 are alike in mineral condition, and they show the same amount of 

 wear. They do not seem to bave been transported far, or subjected 

 to very much rolling about. If any had been washed out of an older 

 cliff, then we should look for more or less difference of wear and of 

 mineral condition between such introduced specimens and the more 

 recent specimens ; but no such distinctions are apparent. All the 

 species, with the exception of the Gyrena, are such as might live in 

 the same sea, and have the ordinary appearance of dead shells cast 

 up on a gravelly bank or on a beach. The Gyrena, however, being a 

 freshwater shell, must either have been washed out of an older deposit 

 or else carried out to sea from the rivers which it inhabited. With 

 this fossil also there is nothing on the score of wear and mineral con- 

 dition to distinguish it from its present marine associates. As these 

 latter indicate a certain amount of transport and intermingling by 

 marine currents, the same cause has in all probability operated along 

 the whole littoral area and caused the intermingling, not only of the 

 shells of different zones of depth, but also of those brought down by 

 streams. That there were such freshwater streams in the vicinity 



* Professor Phillips mentions the occurrence of the tusk of an Elephant in simi- 

 lar beds at Brandesburton (Geology of Yorkshire, 2nd edit. vol. i. p. 22). 



