676 PRINCIPLES OF STRATIGRAPHY 



tensive beds of diatoms are known in various older deposits both 

 marine and lacustrine. Characteristic examples are found in the 

 coastal plain strata of eastern North America, where a single bed 

 underlying the city of Richmond, Virginia, has a thickness of 18 

 feet. Beds of fossil Radiolaria of considerable thickness are also 

 known from older deposits, the most famous being the "Barba- 

 does earth" and the "Tripolite," though this name is often applied 

 to diatomaceous or other siliceous earths. 



Abyssal Deposits of Terrigenous Origin. 



Among these may first be noted the terrigenous matter carried 

 to great distances by the currents, especially opposite the mouths of 

 great rivers. Thus terrigenous deposits are carried out for a thou- 

 sand miles opposite the mouth of the Amazon. The fine coral mud, 

 found around oceanic coral islands, often making the water milky 

 for miles, may settle to the more moderate depths of the abyssal 

 region around these islands. Characteristic deep-sea deposits of 

 continental origin are the blue, green and red clays already referred 

 to, the greensand in the upper portion and the deposits of drift logs 

 and leaves, such as were dredged by the Blake and the Albatross in 

 the Caribbean and off the west coast of America. The most abun- 

 dant deep-sea deposits of terrestrial origin are volcanic ejecta- 

 menta, especially the finer volcanic ashes which are spread far and 

 wide by wind currents and eventually settle on sea and land alike. 

 Even fragments of pumice, dropped on the surface of the ocean 

 and floated until they become waterlogged, are characteristic of 

 deep-sea deposits. Volcanic glass and lapilli are likewise found in 

 the deep-sea deposits, some of these probably originating from sub- 

 marine volcanoes. It is believed that much of the red clay which 

 covers the deeper ocean floor is a result of the decomposition of 

 such volcanic material. This floating volcanic debris is often classed 

 as pelagic, but it is evident that it has nothing in common with the 

 true pelagic material. It may for convenience be classed as pseudo- 

 pelagic, in whicn class also may be included the leaves, tree trunks, 

 etc., which have floated out from the land (pseudoplankton) and 

 come to rest on the bottom of the deep sea. All of these are strictly 

 terrestrial, only the holopelagic and meropelagic types being truly 

 marine. 



TJie Red Clay. 



The red clay of the deep sea is distributed over an area aggre- 

 gating 51,500,000 square miles, and occurs in depths below 2,400 to 



