CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 



19 



Foraminifera, cemented together by a fine calcareous mud, and 

 which was probably formed in water of considerable depth. In the 

 more ordinary limestones — most of which have been formed close to 

 a shore-line — the original accumulation of partially broken-up cal- 

 careous skeletons is subjected to the percolation through its mass of 

 water holding carbonic acid in solution. As the result of this, par- 

 tial solution of the mass takes place, and the dissolved carbonate of 

 lime is ultimately deposited in the form of calcite, the rock thus 



Fig. 3. — Thin section of White Chalk, from 

 Sussex, enlarged about fifty times. The ma- 

 trix is a calcareous mud, and the contained 

 organisms are mostly entire or broken Fora- 

 minifera. (Original.) 



Fig. 4. — Thin section of Carboniferous lime- 

 stone, from Shap, enlarged about fifteen times. 

 The matrix is crystalline, and the included 

 organisms are Foraminifera, calcareous Algae, 

 joints of Crinoids, &c. (Original.) 



assuming the character of a congeries of organic fragments bound 

 together by a general matrix of crystalline carbonate of lime (fig. 4). 

 Even accumulations of sand may be in this way subjected to the per- 

 colation of acidulated water holding lime in solution, and may thus 

 be converted into arenaceous limestones, in which angular quartz- 

 grains are united by a matrix of crystalline carbonate of lime (fig. 2, b). 

 The microscope shows us that very many of the limestones com- 

 posing the crust of the earth, of all geological ages except the most 

 ancient, have been formed in the general method above described. 

 Limestones of essentially similar structure are also now in process of 

 formation on a large scale. This is specially seen in the warm seas 

 of the coral-region, where the broken down coral-sand commonly 

 becomes converted in time into a hard, crystalline or semi-crystalline 

 limestone ; and we may occasionally see the same process at work, 

 on a smaller scale, in the shell-sand of our own shores. 



It follows from the above, that the formation of the crystalline matrix 

 of an ordinary limestone is always secondary to the accumulation of the 



