THE EARTH'S CRUST 



Most limestone deposits are marine formations, and frequently 

 consist largely of shells, but this is not always the case. Small 

 amounts of calcium carbonate are found in many other stratified 

 rocks. 



Impure limestones containing silicate minerals may lose, by 

 weathering and leaching, practically all of the calcium carbonate 

 or magnesium carbonate which they originally contained and leave 

 a residue free from carbonates, as shown by the following analyses: 



TABLE 5. COMPOSITION OF FRESH LIMESTONE AND ITS RESIDUAL CLAY 



Penrose 1 presents convincing evidence that this peculiar man- 

 ganese clay was derived from impure limestone, and Merrill ("Rocks, 

 Rock Weathering, and Soils," 1897, pp. 232, 233) computes that 

 more than 93.6 per cent of the original rock was lost during the 

 processes of decomposition, weathering, and leaching, assuming no 

 loss of silicon. While these analyses represent very satisfactorily 

 the general results of rock weathering, it must not be assumed 

 that the representation is exact in all details. In minor constitu- 

 ents the sample of rock taken for analysis may vary greatly from 

 the particular rock stratum of which the sample of clay was the 

 residue, and the loss by leaching of the same constituent may also 

 vary greatly in different rocks. Thus, in the decomposition of the 



1 Arkansas Geological Survey, Annual Report for 1890, p. 179. 



