Reviews — Dr. L. Cayenx's Sedimentary ItocJcs. 475 



friable. Sponge spicules, and in some instances diatoms, appear to 

 have furnished the soluble silica, which varies in amount from 

 11 to 27 per cent. 



A chapter is devoted to the study of the glauconite so common in 

 the Gaize and Tuffeau. The author brings forward evidence to show 

 that a considerable proportion of this material has been formed long 

 after the deposition of the beds in which it now occurs, and con- 

 sequently that its origin is entirely independent of organic matter. 



Chapter VI contains a description of the Radiolaria occurring in the 

 Smectique de Herve. Though limited in numbers, there is a great 

 variety of forms present, which are placed under twenty-seven genera. 

 The predominant forms belong to the Discoidea, and mainly to the 

 family Porodiscida ; the Cyrtoidea are also well represented. Two 

 or thi'ee new genera are proposed : one of these, Monostyhis, appears 

 to be a synonym of Dorysphcera, Hinde (Ann. and Mag. Nat. Hist., 

 ser. VI, vol. vi, 1890, p. 52). The rock containing these Eadiolaria is 

 regarded as a terrigenous rather than a pelagic deposit, since sand 

 grains are present in it ; the organisms are, however, altogether 

 too few to give the rock any pretension to be radiolarian in 

 character. 



The second part of the volume contains a detailed description 

 of the composition of the Turonian and Senonian Chalk of the 

 Paris Basin. The different areas treated of are : the North, Pays 

 de Bray, Eouen and district, South-East, South- West, West, and 

 North- West of the Basin. The Turonian Chalk is subdivided into 

 the zones of Actinocamax plenus, Inoceramus lahiatus, Terebratulina 

 gracilis, and Micraster breviporiis, and the Senonian into those of 

 Micraster cor-testudinarium, M. cor-anguinum, and the Chalk with 

 Belemnitellas. The composition of the Chalk of the respective zones 

 in each area is given, showing the various kinds of oi'ganic remains, 

 the nature of the minerals in the residues, whether formed in the 

 rock or of clastic derivation, and also the nature of the cement. 



Excepting in the Turonian beds of the Nord and Pays de Bray, 

 the proportion of mineral residues in the Chalk of the Paris Basin — 

 leaving the argillaceous material on one side — is less than 1 per 

 cent. Quartz or sand grains are the most important constituents. 

 The grains range on an average between O'Oimm. and 0'12mm. 

 in diameter, but in all the deposits there are some reaching to 

 0-2-0'4: mm. They are mainly angular, with blunted edges; some 

 are rounded, and a few crystals have been formed in situ. Many 

 other kinds of mineral grains are associated with the quartz, such 

 as zircon, tourmaline, rutile, magnetite, apatite, chlorite, etc. 



Quartzite pebbles (galets) and fragments of schist occur not 

 infrequently in the Senonian beds in the vicinity of Lille. Most 

 of them are between 2 and 8 grams in weight ; the largest noticed 

 weighed 300 grams (=10oz. av.). They resemble some of the 

 primary rocks of the Ardennes. 



The principal secondary minerals formed in the Paris Chalk, in 

 addition to flints, are glauconite, phosphate of lime, calcite, pyrite, 



