Geology of the South-east of Devonshire. 435 



which the rivers of South Devon occasionally exert will probably appear incredible 

 to the casual visitor ; but whoever is interested in such inquiries will be able to col- 

 lect much information in the upper parts of the courses of the Erme, the Dart, and 

 the Teign. 



§ 3. Marine beds. — The accumulations in progress over the bed of the sea vary 

 in character with the depth of the water ; coarse materials being seldom found 

 beyond the ten-fathom line ; from that to forty, the sands decrease in coarseness, and 

 deeper still is only mud or ooze. We know from soundings that in some places 

 the teeth of certain fishes, and that in others certain shells, are peculiar to 

 particular banks, — a grouping of individuals of the same species, of which we find 

 such frequent instances in regular deposits, particularly in the crag. If therefore 

 at the present time very different suites of organic remains are being buried in the 

 contemporaneous accumulations of even inconsiderable areas, we seem hardly war- 

 ranted in making the use we now do of the fossil forms of the older formations. 

 If also we confine our observations to the natural productions of that small por- 

 tion of the coast of the south-west of England which belongs to the district here 

 described, it will be found to present pecuharities which must of necessity be 

 perpetuated. 



In the south-western parts of England the fern tribe bears a larger proportion to the rest of the vegeta- 

 tion than elsewhere ; thus in the valley of the Dart, above Ashburton, the Osmunda regalis occurs in greater 

 abundance than in the whole of the rest of England. It is equally common in certain valleys of the 

 south and west of Ireland. This remarkable fern could not be carried down into the present sea by any 

 streams east of the Teign. 



The teeth of the hake occur in great numbers on certain banks at the opening of the Channel, but 

 from the known range of this Mediterranean fish, its remains will be limited to our south-west shores : 

 in the same manner, about twenty-five other species common in the Mediterranean seldom range further 

 east than the waters of West Bay. So also the testacea which will in future ages characterize these same 

 deposits will contain as many as seventy species common in the Mediterranean ; or in the same manner 

 as the faluns of Touraine differ from the contemporaneous crag of England in the more southern character 

 of its shells, so will the actual deposits off the coasts of Devon and Cornwall differ from those of the 

 Northern and German seas *. 



The following list contains such living shells as are peculiar to the coasts of South Devon and Corn- 

 wall, together with such as are common there, but comparatively rare on the eastern and northern coasts ; 

 the numerical proportion of certain species being, I conceive, in comparisons of this sort, of greater value 

 than the rare occurrence of any particular shell. 



The letters M. P. T. B. mark that the species occurs in the Mediterranean, or fossil in the pliocene 

 deposits of Italy or Sicily, in the faluns of Touraine, or near Bordeaux. 



Spirula australis, Lamk. Anim. sans Vert. 



Bulla aperta, Linn. Syst. Nat. M. 



cylindracea, Pennant, Brit. Zool. M., B. 



Bulla hydatis, Linn. M. 



lignaria, Linn. M., T., B. 



lanthina fragilis, Lamk. 



* See the notice of Mr. Lyell's paper on the Faluns of Touraine, Proceedings Geol. Soc, vol. iii. p. iSV. 



