474 Reviews — Dr. L. Cayeux^s Sedimentary RocJcs. 



Monactinellid, Tetractinellid, and Lithistid sponges abound in tlie 

 beds ; they are now mainly of opal, but some are of clialcedonic 

 silica. Not infrequently the spicules have been dissolved, and 

 their empty casts remain in a cement or groundmass of amorphous 

 silica. 



The Gaize of Argonne in the Cenomanian zone of Amm. inflatns 

 has a thickness of 80 to 105 metres. The amount of soluble silica 

 ranges from 5 to 56 per cent. Sponge-remains in some beds 

 constitute half the rock ; they are of opal, chalcedony, pyrites, 

 glauconite, or merely' hollow casts. There are also a few Eadiolaria, 

 some doubtful diatoms, and, where lime is present, a few Foraminifera, 

 belonging to Textularia, Olohigerina, etc. The cement is mostly of 

 colloid silica, frequently in the condition of minute globules, like 

 those in the sponge-rock of the Upper Greensand of this country. 



The Gaize of the zone of Aram. Mantelli in the Department of 

 Cher, is very similar to that of Argonne. The soluble silica in 

 it varies from 15 to 35 per cent. ; the sponge-remains constitute 

 from one-tenth to one-eighth of the rock, and there are likewise 

 a few Eadiolaria. The cement of this Gaize is also largely of 

 globular colloid silica. 



In the Gaizes above mentioned sponge spicules form the essential 

 element. They vary considerably in numbers in difierent beds : in 

 some they are estimated to form one-half, in others not more than 

 one-tenth of the rock. Radiolaria and diatoms are present, but in 

 very insignificant proportions, and occasionally a few Foraminifera. 

 In the softer and friable kinds of Gaize the silica of the cement or 

 groundmass is colloid in character, but in the harder kinds it is in 

 the form of chalcedony. The author does not consider that the 

 silica of the cement in the -Gaize is entirely due to sponge remains, 

 but that a certain proportion of it is derived from the decomposition 

 of the argillaceous constituents of the rock ; the evidence for this, 

 however, is not by any means convincing. 



The Meule de Bracquegnies, near Mons, belonging to the zone of 

 Amm. itijlatus, is practically of the same character as the Gaize of 

 Argonne. The soluble silica ranges up to 25 per cent. ; some beds 

 are nearly wholly composed of sponge debris, in others this forms 

 about one-half the rock. 



In the vicinity of Liege and in the district of Herve (Belgium) 

 the rock known as Smectique, belonging to the zone of B. quadr-ata 

 (=: Upper Chalk), consists of marls and glauconitic sands from 

 20 to SOm. in thickness. It contains 15 per cent, of soluble 

 silica. The rock is rich in sponge remains, now converted into 

 chalcedony ; it has also some well-preserved Eadiolaria, a few 

 diatoms, and a considerable number of Foraminifera. Unlike typical 

 Gaize, the cement in this rock is mainly calcareous. 



The deposits of ' Tuffeau ' of Eocene age which occur in the 

 North of France and in Belgium are very similar in character to the 

 Gaize and Meule. They may be described as greenish or greyish 

 glauconitic sands vpith argillaceous and calcareous materials, and 

 a cement of soluble silica. Some beds are hard and tenacious, others 



