70 THE WONDERS OF GEOLOGY. Lect. I. 



however, have thrown light upon the deposits now forming 

 in those depths which are accessible to this mode of inves- 

 tigation, and shown that in many parts immense accumula- 

 tions of shells and corals, intermixed with sand, gravel, and 

 mud, are going on. Donati ascertained the existence of a 

 compact bed of shells, 100 feet in thickness, at the bottom 

 of the Adriatic, which in some parts was converted into 

 marble. In the British Channel, extensive banks of sand, 

 imbedding shells, Crustacea, &c. are in the progress of form- 

 ation. This specimen, which was dredged up a few miles 

 off Brighton, is an aggregation of sand with recent marine 

 shells, oysters, mussels, limpets, cockles, &c. and minute 

 corallines ; * and this example, from the coast of the Isle of 

 Sheppey, consists entirely of cockles ( Cardium edule), held 

 together by conglomerated sand.f 



In bays and creeks, bounded by granitic rocks, the bottom 

 is found to be composed of micaceous and quartzose sand, 

 consolidated into what may be termed regenerated granite. 

 Off Cape Frio, on the Brazilian coast, solid masses of this 

 kind were formed in a few months, and in them were found 

 imbedded, dollars and other treasures from the wreck of a 

 vessel, the Thetis, to recover which an exploration by the 

 diving bell was undertaken. 



31. Effects of currents. — The distribution over the 

 bottom of the sea of the detritus brought down by rivers 

 and streams, and of the materials worn away by the action 

 of the waves on the shores, is principally effected by the 

 influence of currents, which, from their regularity, perma- 

 nency, and extent, may be considered as the rivers of the 

 ocean. To this agency I can but briefly allude, and shall 

 restrict my remarks to the Gulf-stream, which is the great 

 current that transports the waters, and the temperature of 



* Medals of Creation, vol. i. p. 375, lign. 87. 

 j Ibid. p. 377, lign. 88. 



