334 
SECONDARY FORMATIONS 
[Ch. XXIII. 
Secondary rocks why 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, are 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 m 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 setting 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 1 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. 
