Reviews — Br. L. Cayeux's Sedimentary Rocks. 477 



spicules and the scarcity of Hexactinellid forms ; whilst, on 

 the other hand, the zone of M. breviporus is characterized 

 by Monactinellid and Tetractinellid spicules. In the Senonian, 

 Hexactinellid spicules appear in the residues. Detached spicules 

 of Calcisponges are found in all the Chalk beds. 



The remains of siliceous sponges are now only exceptionally 

 preserved in colloid silica ; most commonly they are replaced by 

 calcite or by glauconite, and more rai'ely by pyrites, phosphate of 

 lime, or by limonite. The author notes the rarity of empty casts 

 of spicules in the Chalk as compared with those in the Gaize, but 

 this may be less than appears, since the empty casts are very 

 inconspicuous in the Chalk, and careful observation with a lens is 

 needed to distinguish them. 



The author also calls attention to the fact that Lithistid spicules 

 are not replaced by glauconite the same as spicules of other groups 

 of sponges in the Chalk. The explanation of this appears to be that 

 the replacement of siliceous spicules by glauconite is effected by way 

 of infilling of their axial canals ; and as in the skeletal spicules of 

 Lithistids the axial canals are, as a rule, very slightly, if at all, 

 developed, replacement by glauconite does not occur. The spicules 

 (so termed) in nearly all the groups from the Chalk figured by the 

 author are, in fact, merely the solid infilling by glauconite of the 

 enlarged axial canals of genuine spicules : this is shown by the even 

 thickness and the truncation of the ends of the spicular rays. The 

 contrast between these glauconitic replacements and actual spicules 

 may be seen in the figures of a group of the latter (fig. 13, p. 290) 

 from the upper zone of the Turonian Chalk of Rouen. 



A few Eadiolaria have been observed at different horizons in the 

 Chalk ; they are more numerous in the Senonian. In some instances 

 they retain their siliceous structure ; in others this has been 

 replaced by calcite or phosphate of lime. 



The proportion of Foraminifera in the Chalk examined varied 

 from 5 to 80 per cent. ; the maximum amount was found in the 

 Turonian Chalk of the Rouen district. Forms of Glohigerina are 

 stated to play a subordinate role as compared with those of Textii- 

 laria and Botnlia. The thickness of the foraminiferal tests varies 

 considerably in different beds, and probably indicates variations of 

 depth in the seas of the period. 



Diatoms have been but rarely observed ; on the other hand, 

 Coccoliths and Rhabdoliths are present everywhei'e, the former by 

 far the most numerous. The author regards them as pelagic Algge. 



The cement or matrix of the Paris Chalk is composed of fine 

 particles derived from the breaking up of the various organisms, the 

 microscopic Algse, and crystals of calcite ; it varies in amount from 

 one-tenth to nine-tenths in different beds and localities. In every 

 sample of chalk examined the organisms present show traces of 

 dissolution, and there is no evidence of any direct chemical deposi- 

 tion of carbonate of lime from the sea- water. 



The author unhesitatingly considers the typical Chalk of the Paris 

 Basin, such as that of the Pays de Bray, the Rouen district, and the 



