No. 136. J 27 



occur in a certain position in the series, such as coal, or some 

 beds of iron ore. 



Yet the chief interest of fossils is in their historic value, as 

 giving us records of the condition of earth and sea and their 

 inhabitants at the different periods of the vast succession of ages 

 during which all the stratified rocks known were deposited ; of 

 which, much will appear in the detailed account hereafter given 

 of the separate rocky strata. 



These relics are found in a great variety of conditions. Shells 

 are found with both valves united as when living, either open or 

 closed ; but more frequently with the hinge broken and the 

 valves separated. Sometimes they are hardly changed from 

 their original texture; at other times, quite " petrified," or re- 

 placed by stone like the 'surrounding rock : in some cases they 

 are converted into iron pyrites ; in others, into white calcareous 

 spar, with crystals of which their cavities are often lined or 

 filled. Often the shell has decomposed, leaving in the rock only 

 a hollow marked with its external impression : sometimes we find 

 a cast or mould of its interior, showing the forms of the teeth of 

 the hinge, and the pits or other marks where the inhabiting 

 animal was attached to it. Often they are distorted or com- 

 pressed even to flatness, by the consolidating or settling of the 

 material in which they were imbedded. (This is especially com- 

 mon in slaty rocks ; while in limestones, which would seem to 

 have hardened at once when first deposited, shells are usually 

 not compressed.) 



Other fossils occur in various conditions. Fishes are rarely 

 perfect, the skeleton being generally separated and the bones and 

 scales more or less scattered. Crustaceans, or animals with 

 jointed coverings like the lobsters or crabs, are also usually found 

 to have been disjointed by decay before they were buried in the 

 sediment, so that specimens in an entire state are not common, 

 though detached plates are abundant. The stems of plants are 

 usually much flattened, and often converted into coally matter ; 

 while leaves generally show as mere imprints in the stone, black- 

 ened by the carbonaceous remains of their substance. 



From this brief notice of the origin of the stratified fossil- 

 bearing rocks and the nature of their organic relics, we may be 

 prepared to appreciate the value to the cause of science (which 

 is only another name for accurate knowledge) of a thorough 

 investigation of a series of these rocks and of their fossil con- 



