METAMORPHISM OF CALCIUM CARBONATE DEPOSITS. 797 



Further, it has been explained that in the belt of weathering solution 

 is the rule, and that in the belt of cementation solution does not so far surpass 

 deposition but that cementation is the rule. No other rock so well illus- 

 trates these principles as limestone, for it is the most readily soluble of the 

 extensive formations. Under the law of dominant solution in the belt of 

 weathering- the openings are enlarged, and thus the very numerous cracks, 

 crevices, and caves, great and small, so characteristic of limestone, are 

 formed. But while solution dominates in the belt of weathering, as shown 

 by these openings, deposition is, as explained on page 487, an invariably 

 accompanying process. Thus are made the stalactites, stalagmites, and 

 travertine deposits. These deposits may be either aragonite or calcite. 



In the belt of cementation below the level of ground water in lime- 

 stones the openings formed in various ways are as generally decreased in 

 size by deposition as they are enlarged in the belt of weathering. The 

 material here deposited is generally in the stable form of calcite. 



But the process of solution and deposition in the belt of weathering 

 and the belt of cementation does not affect the walls of the openings only. 

 Solution and deposition are going on continuously throughout the entire 

 mass of limestone formations in the zone of katamorphism. As a conse- 

 quence of this the rock is continuously recrystallized. This process, even 

 under mass-static conditions, may advance so far as to transform a limestone 

 into a uniformly crystalline rock, to which the term marble is applicable. 

 However, where the recrystallization goes so far as to transform the rock 

 to a marble, there has usually been more or less mechanical action. But 

 the consideration of the marbles is deferred. (See pp. 808-816.) 



In modern limestones fossils are likely to be abundant; in ancient 

 limestones they are comparatively sjmrse. In general it may be said that 

 they are less abundant as the rocks are old. Also, in proportion as the 

 rocks are old they are likely to be well crystallized. The increased sparse- 

 ness of fossil remains and the crystalline character of the limestone may both 

 be regarded as evidence that recrystallization has taken place. Also they 

 might be interpreted as evidence that life was not so abundant in the past 

 and that the material had been deposited as a chemical precipitate. But cer- 

 tainly the latter argument does not apply to rocks later than the Algonkian. 

 Since fossils so commonly constitute but a small fraction of the present 

 mass of limestones, it is concluded that recrystallization has extensively 

 occurred in Paleozoic and post-Paleozoic limestones. (See PI. VI, A.) 



