Reviews — Dr. L. Cayeux's Sedimentary Rocks. 473 



have been subjected, and thus to gain an idea of their original 

 structure and of the conditions under which they were formed. 

 The work has been carried on by means of microscopic sections, 

 chemical analyses, and more particularly by the study of the residues 

 after treatment of the calcareous rocks with acid. 



The first part of the volume contains a description of the 

 siliceous deposits known in France and Belgium under the names 

 of 'Gaize, ' 'Meule, ' ' Sraectique, ' ' Tetes de Chat,' 'Rabots, ' 

 and ' Tufieau.' Other synonyms of the Gaize are * Gres Vert,' 

 ' Craie tufau,' ' Pierre morte,' and ' Pougzolane.' Typical Gaize is 

 a soft, porous, dirty gray or yellow siliceous rock of a sandy texture, 

 with a varying amount of soluble silica in its composition. It 

 frequently contains harder compact nodules, comparable in some 

 respects to chalk flints, but unlike them in not being sharply 

 delimitated from the softer matrix. It is as a rule rich in the 

 debris of siliceous organisms, with variable amounts of quartz 

 (sand-grains) and glauconite, also in some instances a small 

 proportion of carbonate of lime. These constituents are usually 

 cemented together by opalized or by chalcedonic silica. It may 

 be said in passing that the ' Gaize ' corresponds with those siliceous 

 beds in the Lower and Upper Greensand of this country generally 

 referred to as ' Chert,' ' Malm,' ' Sponge-rock,' etc. 



Deposits of Gaize are prominently developed on three distinct 

 horizons in France : (1) in the Oxfordian beds of the Middle 

 Jurassic Series in the Ardennes ; (2) in the Albian (= Lower 

 Gault), also in the Ardennes ; and (3) in the Cenomanian 

 (z=. Upper Greensand) in Argonne and Pays de Bray. 



The Jurassic Gaize, in the zone of Amm. ( Cardioceras) Lamberti 

 and Amm. Marice, is only known in the Ardennes, where it attains 

 a thickness of about 50 metres. The soluble silica varies in 

 difierent specimens from 9 to 56 per cent., and glauconite 

 forms one-tenth of the rock. The organic constituents are princi- 

 pally minute rounded, oval or kidney-shaped, siliceous bodies, 

 which are sufficiently numerous to constitute one-third to one-half 

 of the rock, and with these are fractured siliceous sponge spicules. 

 In some examples both the matrix and the rounded bodies are 

 replaced by calcite. The author recognizes the similarity of these 

 rounded and reniform bodies to those first described by Dr. Sorby 

 from the corresponding horizon in the Calcareous Grit of Yorkshire, 

 but he does not consider them to belong to Geodia sponges, owing to 

 the absence of the corresponding skeletal spicules, and to the hollow 

 condition in which many of these bodies now occur. It has, how- 

 ever, been shown that definite sponges occur in the Yorkshire beds 

 apparently wholly made up of these peculiar rounded bodies (Quart. 

 Journ. Geol. Soc, vol. xlvi, 1890, p. 54), a fact which Dr. Cayeux 

 seems to have overlooked. 



The Gaize of the Albian zones of Amvi. mammillaris and Amm. 

 interruptus is partly a soft porous rock, partly a coarse glauconitic 

 grit ; one sample yielded 28 per cent, of soluble silica. Spicules of 



