194 



STRATIFIED OR SEDIMENTARY ROCKS. 



stone ; b and d, especially the latter, are found in all kinds of rocks. 

 By far the most common infiltration fillings are carbonate of lime and 

 silica. 



Often we find impressions of the forms of small portions only of the 

 original organism, as of the leaves of trees, or the feet of animals walk- 



Fig. 176.— a. Cast of interior 

 b, natural form. 



Fig. 177.— a, Natural form; Fig. 178.— Trigonia Longa, showing 

 b, cast of interior and cast (a) of the exterior and {b) of 



mold of exterior. 



the interior of the shell. 



ing on the soft mud of the flat shores of ancient bays. Such tracks 

 were afterward covered up with river or tidal deposit, and thus pre- 

 served. On cleaving the rock along the lamination-planes we have on 

 one side a mold and on the other the cast of the foot 



Between cases 1 and 2 every stage of gradation may be traced. 

 The amount of change, as a general fact, varies with the age of the 

 rock ; but is still more dependent on the kind of rock and the degree 

 of metamorphism. In an impermeable rock, like clay, the changes are 

 much more slow than in a porous rock, like sandstone. 



Distribution of Fossils in the Strata. 



The nature of the fossil species found in rocks is determined partly 

 by the hind of rock, partly by the country where the rock is found, and 

 partly by the age of the rock. 



1. Kind of Rock. — It is well known that the species of lower marine 

 animals vary with the depth. They also vary with the kind of bottom. 

 Thus, along shore-lines and on sand-bottom the species differ from 

 those in deep water and on mud-bottom. Shells are found mostly 

 along shore-lines, corals in opener seas, and foraminifera in deep seas. 

 The same was true in every previous epoch. We might expect, there- 

 fore, and do find, that the lower marine fossils of sandstones, shales, 

 and limestones, differ even when these strata belong to the same coun- 

 try and geological epoch. The higher marine animals, such as fishes, 

 cuttle-fish, etc., swimming freely in the sea, are more independent of 

 bottoms, and we find their skeletons and shells equally in all kinds of 

 strata. Land animals perish on land, and their skeletons are drifted 

 into bays, river-deltas, and lakes, and buried there mostly in fresh- 



