SHELL-DEPOSITS. 161 



running through limestone countries, the excess may be deposited as a 

 chemical deposit. But in most cases sea- water contains less lime-car- 

 bonate than river- water. The reason is, that the lime-carbonate in sea- 

 water is continually being drafted upon by organisms and deposited on 

 their death as organic limestones. We have already shown how coral 

 limestone is thus formed. But there are many other limestone-forming 

 animals, and some species form other kinds of deposits besides lime- 

 stone. 



Molluscous Shells, — Shallow-water deposits of this kind are made 

 principally by mollusca which, living in immense numbers near shore 

 and on submarine banks, leave their dead shells generation after gener- 

 ation, and thus form sometimes pure shelly deposits, and sometimes 

 shells mingled with sediments due to other agencies. On quiet shores 

 the shells are quite perfect, whether imbedded in mud or forming shell- 

 banks like our oyster-banks ; but when exposed to the action of break- 

 ers, they are broken into coarse fragments, or even comminuted, worn 

 into rounded granules, and cemented into shell-rock or oolitic rock. 

 Such shell-rock and oolitic rock are now being formed on the coast of 

 the Florida keys and of the West Indies. Similar rock is found in 

 every part of the world in the interior of continents. They indicate 

 the existence in these places of a shore-line or of shallow water in some 

 previous geological epoch. 



Microscopic Shells. — Microscopic plants and animals are known to 

 multiply in numbers with almost incredible rapidity. Many of them 

 form rio shell, and therefore are of no geological importance ; but many 

 species form shells of silica or of carbonate of lime, and these of course 

 accumulate generation after generation, until important deposits are 

 formed. 



Fresh-water Deposits. — In streams, ponds, lakes, and hot springs, 

 the beautiful siliceous shells of diatoms (uni-celled plants) accumulate 

 without limit. The ooze at the bottom of clear ponds, or lakes, as, for 

 example, in the deepest parts of Lake Tahoe, consists often wholly of 

 these shells. Diatoms live also in great numbers in the hot springs of 

 California, Nevada, and Yellowstone Park, and the deposits of such 

 sprinjs sometimes consist wholly of these shells, and in .Yellowstone 

 Park cover many square miles, and are five to six feet thick.* Thick 

 strata, belonging to earlier geological times, are found wholly composed 

 of diatoms. We are thus able to explain the formation of these strata. 



Beep-sea Deposits. — Over nearly all the bottom of deep seas, be- 

 yond the reach of sedimentary deposits, we find a white, sticky ooze, 

 composed of the carbonate-of-lime shells of microscopic animals (fora- 

 minifers), Fig. 129, and microscopic plants (coccospheres). Some of 



* Weed, Botanical Gazette, May, 1889. 

 11 



