THE CAMBRIDGE GAULT AND GREENLAND. 275 



the original calcareous shell is often well preserved. The Crustacea 

 frequently retain their tests, apparently in a phosphatized condition; 

 but the limbs are seldom found attached. The Echinoderms and 

 Corals nearly always occur as waterworn casts, the former only 

 occasionally retaining remnants of their calcareous shell. 



Finally, it may be noticed that these dark-coloured fossils and 

 nodules are always met with at the very bottom of the bed, and are 

 much covered with small Plicatulce, Ostrece, and Spondyli. 



The second group of fossils (see list, p. 311) comprises specimens 

 which are obviously of the same age as the bed itself, and are easily 

 separated from the former by the difference of their mineral con- 

 dition. 



The bones are more perfect, lighter in colour, and their interstices 

 are usually filled with the sandy marl of the formation ; this causes 

 them to be more friable, and when they have been washed in the 

 mills they often appear as rolled and broken as their darker com- 

 panions ; but this is not the case when they are taken fresh from the 

 deposit itself. 



The Mollusca of this fauna are chiefly Lamellibranchs and Bra- 

 chiopoda, and in all cases the calcareous shells are preserved ; their 

 interiors are either filled with indurated sandy marl, or with a sub- 

 stance which is apparently half calcareous half phosphatic in its 

 composition ; and it is these lighter phosphates which range upwards 

 into the Chalk-marl. 



Amongst the lesser debris several small Brachiopods, Corals, 

 Echinoderm-spines, and Eoraminifera may always be discovered. 



Finally, it may be remarked that many of the species are essen- 

 tially characteristic of the lower beds of the Chalk. 



For further information regarding the condition of the various 

 organic remains I may refer to Mr. Bonney's paper previously cited, 

 and to papers by Mr. Sollas in the Quart. Journ. Geol. Soc. vol. xxviii. 

 p. 397, and Geol. Mag. vol. x. p. 268. I may remark, however, that 

 the shells of the Ostreidae seem to possess some peculiarity of struc- 

 ture or composition which enables them to resist wear and tear, as 

 well as dissolving agents, much better than others. The shells of 

 those in the nodule-bed are almost always preserved, and though 

 many of them are of the same age as the deposit, others which are 

 certainly derived still retain their shell in a more or less good state 

 of preservation ; consequently in some cases it is difficult to say to 

 which fauna a given individual most probably belongs. The internal 

 cast is a good criterion, and when that consists of clean dark phos- 

 phate there is no doubt of its derived origin. The cases of light 

 phosphate with black phosphate pebbles are more ambiguous. 



Both Mr. Bonney and Mr. Walker have noticed the presence of two 

 distinct faunas in the formation, the former at p. 13 of his paper on 

 the subject, and the latter in a note to one of Mr. Davidson's papers 

 on Continental Geology. Mr. Walker here gives his reasons for 

 believing that the Cambridge bed has resulted from the denudation 

 of the Gault ; and after remarking upon the waterworn character of 

 the fossils, the rareness of associated bones, and the frequent presence 



Q.J.G.S. No. 122. tt 



