CHAP. XII.] ICENIAN PERIOD. 241 



these sands, it is probable that they are nearly of the same 

 age, though possibly deposited in somewhat deeper water. 

 At their base is a pebble bed which contains the " box- 

 stones " already referred to, as well as rolled bones of 

 various mammalia, pebbles of flint and sandstone, blocks 

 of septaria from the London Clay, and at Sutton a large 

 boulder of dark red felstone. The overlying sands and 

 marls are from 20 to 50 feet thick ; they contain numerous 

 shells and Bryozoa, some in the position of life and growth, 

 others drifted and mingled with layers of comminuted 

 shells. 



According to D'Orbigny the conditions essential to the 

 growth of Bryozoa are a considerable depth of water, clear 

 water, and the existence of strong currents ; the modern 

 species of Lepralia, for instance, of which many occur in the 

 Crag, are generally found between 40 and 150 fathoms. 1 

 Of the Corals one species lives now at depths of 10 to 60 

 fathoms, and the descendant of another (Fldbellum Woodii) 

 has only been found at depths of over 300 fathoms. The 

 Foraminifera, according to Messrs. Jones, Parker, and 

 Brady, agree best with the assemblage found between 50 

 and 70 fathoms off the Scilly Isles. 



The sandy beds are overlain by the peculiar rock which 

 has given the name of Coralline Crag to the group ; this is 

 a soft, porous, yellow calcareous rock, which is almost en- 

 tirely composed of comminuted shells and Bryozoa ; it is 

 from 30 to 40 feet thick, and the irregular bedding and 

 oblique lamination which it exhibits are proofs of the action 

 of strong currents in drifting, sifting, and reconstructing 

 the layers of which it is composed. Mr. Prestwich con- 

 cludes, therefore, that this rock indicates shallower water 

 than that in which the sandy marls below were deposited, 

 and infers that during its formation the sea-floor was raised 



Prestwich, " Quart. Journ. Geol. Soc.," vol. xxvii. p. 129. 



J. J, t O L IT H.J.A ^ VU.t*A I* WUAU 



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