476 Reviews — Dr. L. Cayeux's Sedimentary Rocks. 



limonite, manganese, quartz, opal, etc. Glauconite occurs throuojli- 

 out, both as casts of organisms and as independent grains. The 

 phosphate of lime is either amorphous or crystalline ; some of the 

 grains are free from all connection v/ith organisms and formed in 

 place. The nodules of this material have a strong preservative 

 influence on the organisms, whether calcareous or siliceous, vi^hich 

 they inclose. They are more numerous in beds which indicate some 

 disturbance or interruption of the normal conditions of deposition. 



The calcite in the Chalk occurs in isolated rhombohedric crystals, 

 which are very generally distributed; these are oftentimes dissolved, 

 leaving perfect geometrical cavities. This dissolution frequently 

 takes place in an apparently capricious manner ; certain bands of 

 Chalk retaining the crystals, whilst in alternating bands they are 

 completely removed. The i"ock with the hollow casts is usually 

 soft and friable, that with the crystals hard and durable ; and in 

 beds where the calcite crystals have been partially dissolved, the 

 portions intact appear as nodular masses inclosed in a soft matrix. 



With respect to the flints, the author considers that they may 

 have been formed at several periods in the history of the Chalk in 

 which they now occur, and, further, that in the Paris Basin the 

 amount of silica they represent bears a close relation to the number 

 and volume of the sponge spicules in the same beds which have been 

 replaced by calcite. The rarity or absence of flints in the Chalk 

 cannot be taken as a reliable index of the part played by sponges in 

 the particular beds, for the silica of the sponge remains may be 

 dispersed through the Chalk in the form of minute colloidal globules, 

 instead of being aggregated into flints, or it may have been carried 

 by solution into lower beds. 



Of the organic remains in the Paris Chalk, the debris of Molluscan 

 and Brachiopod shells occurs throughout ; more particularly is 

 this the case with the detached prisms of the shells of Inoceramus, 

 which in certain beds near Lille, at the top of the Turonian and at 

 the base of the Senonian, are sufficiently numerous to form nine- 

 tenths of the Chalk. 



Polyzoa are very largely developed in some of the Turonian 

 Chalks of the South- West of the Basin, where even the finer particles 

 of the beds are mainly composed of their comminuted remains, 

 Echinoderm fragments are present at all horizons, but they are less 

 abundant in the Turonian than in the lower part of the Senonian. 

 Corals play an uncertain part, possibly on account of their aragonitie 

 character. 



Though detached sponge spicules occur throughout the Chalk of 

 the Paris Basin, it is necessary to dissolve a considerable amount 

 of the Chalk to obtain them in the residues. Occasionally large 

 numbers are present: for instance, in some beds at Meudon they are 

 estimated to form one-fifth of the rock, and in the 31. cor-anguinum 

 zone at Maintenon, one-fourth. The most constant horizon for 

 sponge-remains in the Paris Basin is at the summit of the Turonian. 

 The I. lahiatm and T. gracilis beds of the South-West and West 

 of the Basin are distinguished by the abundance of Lithistid 



