SECONDARY FORMATIONS [Ch. XXIII. 



Secondary rocks ivhy more consolidated. One of the points 

 where the analogy between the secondary and tertiary forma- 

 tions has been supposed to fail is the greater degree of solidity 

 observable in the former. Undoubtedly the older rocks, in 

 general, arc more stony than the newer ; and most of the ter- 

 tiary strata are more loose and incoherent in their texture than 

 the secondary. Many exceptions, however, may be pointed 

 out, especially in those calcareous and siliceous deposits which 

 have been precipitated in great part from the waters of mineral 

 springs, and have been originally compact. Of this description 

 are a large proportion of the Parisian Eocene rocks, which are 

 more stony than most of the English secondary groups. 



But a great number of strata have evidently been consoli- 

 dated subsequently to their deposition by a slow lapidifying 

 process. Thus loose sand and gravel are bound together by 

 waters holding carbonate and oxide of iron, carbonate of lime,, 

 silica, and other ingredients, in solution. These waters per- 

 colate slowly the earth's crust in different regions, and often 

 remove gradually the component elements of fossil organic 

 bodies, substituting other substances in their place. It seems., 

 moreover, that the draining off of the waters during the 

 elevation of land may often cause the seltiny of particular 

 mixtures, in the same manner as mortar hardens when desic- 

 cated, or as the recent soft marl of Lake Superior becomes 

 highly indurated when exposed to the air *. The conver- 

 sion of clay into shale, and of sand into sandstone, may, in 

 many cases, be attributed to simple pressure, produced by the 

 weight of superincumbent strata, or by the upward heaving 

 of subjacent masses during earthquakes. Heat is another 

 cause of a more compact and crystalline texture, which will 

 be considered when we speak of the strata termed ' primary/ 

 All the changes produced by these various means require time 

 for their completion ; and this may explain, in a satisfactory 

 manner, why the older rocks are most consolidated, without 



* Vol. i. p. 226, and Second Edition, p. 259. 



