198 THE OCEAN 



sented by only a few decomposing frag- 

 ments. 



These various inorganic and organic con- 

 stituents of marine deposits are present in 

 the samples in very different proportions, 

 varying according to the distance from shore, 

 the depth of water, the latitude, and the 

 physical and chemical conditions of the surface 

 waters. There is a wide distinction between 

 a typical example of a globigerina ooze and 

 one of red clay, or between a pteropod ooze 

 and a diatom ooze, or a blue mud or green 

 sand and a radiolarian ooze, but these varieties 

 may merge by numerous gradations the one 

 into the other. 



Marine deposits might be classified in a 

 great many ways, but after much considera- 

 tion it has been found that a combined chemi- 

 cal and microscopical analysis gives the 

 best results, and is the most useful for the 

 geologist and the physical geographer. The 

 microscope shows us that the calcium car- 

 bonate in a deposit consists mainly of dead 

 shells of organisms, and we can tell whether 

 these have lived on the surface of the sea or on 

 the bottom, as well as the orders, genera and 

 species to which they belong. In the same way 

 the source of the siliceous remains of organisms 

 and of the mineral particles can be determined. 

 It is easy to determine the percentage of 



