386 . GEOLOGY. 



monly known as green sand marl. Glauconite also occurs sparingly in 

 many other sedimentary rocks. : j, 



Another substance which is somewhat widespread in the ocean^ 

 bottom is phosphate of lime, which occurs in various sorts of oozes, in 

 the manganiferous nodules, in glauconite, and in independent nodulesl 

 Like the grains of glauconite, the grains of phosphate of lime appeal 

 to have started as concretions in shells, and to be the result of the reaction 

 of organic matter on the contents of sea-water. The immediate source 

 of the lime phosphate in the water appears to have been the shells or 

 bones of the numerous animals living in the sea. 



Secondary minerals made from the constituents of volcanic mat- 

 ter which has been decomposed occur not uncommonly in the bottom 

 of the sea. These minerals belong to the general class of zeolites, phil- 

 lipsite being the most abundant. Their distribution is somewhat 

 wide, but their quantity is slight. 



Unfortunately, knowledge of the deep-sea deposits is limited to 

 their superficial lajyers. Soundings do not usually penetrate more thari 

 a few inches, or^tj,most a foot or two. 



Unlike shallb#-water deposits, those of the really deep sea seem 

 to find no correlatives in the known rock formations of the land. 



LAKES. 



Most of the phenomena of the ocean are repeated on a smaller scale 

 in lakes. The waves of lakes and their attendant undertows and littoral 

 currents are governed by the same laws and do the same sort of work 

 as the corresponding movements of the ocean. Tides are absent, or 

 insignificant, but slight changes of level, known as seiches,^ have been 

 observed in many lakes. They are probably caused by sudden changes 

 in atmospheric pressure. While they are generally very shght, they 

 frequently amount to as much as a foot, and occasionally to several 

 feet. The seiches are oscillatory movements, and their period is in- 

 fluenced by the length and depth of the lake. They have been studied 

 most carefully in Switzerland. Currents corresponding to those of the 

 ocean are slight or wanting in lakes, but since most lakes have inlets 

 and outlets, their waters are in constant movement toward the latter. 

 In most cases this movement is too slow to be readily noted, or to do 

 effective work either in corrasion or transportation. The work of the 



1 Forel, Compte Rendu, 1875, 1876, 1878, 1879, and P. Du Bois, 1891. Also Forel's 

 Lac Leman. 



