378 GEOLOGY. 



rapid, as it often is in shallow water, this action is Hmited and obscure. 

 In the main, the ocean-waters protect the sediments from weathering 

 and similar changes, except as organic matter buried with them induces 

 change. 



While the hme deposits are by far the greatest of the chemical and 

 organic deposits of the sea, plants and animals also secrete notable 

 quantities of sihca. Sihca deposits of organic origin are relatively 

 much more important in the deep sea than in shallow water, and will 

 be mentioned in that connection. 



Limestone. — Something concerning the origin of limestone has already 

 been given in the preceding paragraphs, but because of the importance 

 of this formation, it may be added by way of summary that shallow seas 

 free, or nearly free, from terrigenous sediment, and abounding in 

 Hme-secreting life, furnish the conditions for nearly pure deposits of 

 limestone, and that most of the Hmestone within the areas of the 

 present continents appears to have originated under such conditions. 

 The common notion that limestone is normally a deep-water formation 

 is a serious error. Although limestones are formed in deep as well as in 

 shallow waters, by far the more important classes of lime-secreting 

 organisms are photo-bathic, i.e. are limited to the depths to which hght 

 penetrates. In the shallow waters, these plants and animals are in part 

 free and in part attached. Within the areas of deep water they are 

 free and at the surface, and their remains drop to the bottom, if not sooner 

 dissolved. But few forms live on the deep, dark, cold bottoms of abys- 

 mal depths. Clear waters, free from abundant terrigenous sediments 

 and abounding in lime-secreting life, rather than deep waters, are, 

 therefore, the most favorable conditions for the origin of limestone. 



The purely chemical deposits of hmestone are probably all of shallow- 

 water origin. Once made, they are subject to solution, re-deposition, 

 aad: other mutations like other deposits. As a result, they often lose 

 many of their original characteristics, but enough usually remain to 

 tell the story of their origin. 



Deep-sea Deposits, 



Contrasted with shallow-water deposits. — The deep-sea deposits cover 

 the ocean-bottom below the 100-fathom line. Their area is consider- 

 ably more than half the earth's surface. The characteristic deposits 



