1 6 Limestones. 



No. 2 ; and it is equally common to find large quantities 

 of shells, corals, sea-urchins, encrinites, and various 

 other forms of life in such limestones as No. 1, which, 

 in many cases, are almost wholly composed of entire 

 or broken shells and other marine organic remains. 



Marine and lake sediments form soils on and in 

 which the creatures live that inhabit the bottom of the 

 waters, and it is easy to understand how numerous 

 shells and other organic bodies happen thus to have 

 been buried in muddy, sandy, or conglomeratic 

 mechanical sediments, the component grains of which, 

 large or small, have been borne from the land into 

 water, there by force of gravitation to arrange them- 

 selves as strata. By the life and death of shells in 

 these fossilised sediments, it is also easy to understand 

 why they are so often more or less calcareous. The 

 question, however, arises, how it happens that strata 

 of pure or nearly pure carbonate of lime or limestone 

 have been formed. 



Though the materials of shale (once mud), sand- 

 stone (once loose sand), and conglomerate (once loose 

 pebbles), have been carried from the land into the sea, 

 and there arranged as strata, and though limestones 

 have, in great part, been also mechanically arranged, 

 yet it comparatively rarely happens that quantities of 

 fine unmixed calcareous sediment have been carried in 

 a tangible form by rivers to the sea, though it has some- 

 times been directly derived from the waste of sea-cliffs, 

 and mixed with other marine sediments. When, there- 

 fore, it so happens that we get a mass of limestone 

 consisting entirely of shells, corals, and other remains, 

 which are the skeletons of creatures that lived in the 

 sea, in estuaries, or in lakes, the conclusion is forced 

 upon US' that, be the limestone ever so thick, it has been 



