i;8 



MARINE DEPOSITS 



molluscs, nullipores, etc.). The deposit is purest and most 

 typical in the medium depths of the ocean, far from any land ; in 

 such places the ooze may contain as much as 90% CaCO s and is 

 white, while nearer land the slight admixture of terrigenous minerals 

 gives a pink, grey, brown, or other colour to the mass. Below the 

 depth of 2500 fathoms the proportion of CaC0 3 becomes much 

 diminished, owing to the increasing percentage of C0 2 in the sea- 

 water, which attacks and dissolves these delicate shells. 



The foraminiferal oozes have a vast geographical extent, esti- 

 mated at 49,520,000 square miles, and are especially developed in 



FIG. 73. — Foraminiferal ooze. X 20. (Agassiz after Murray and Renard.) 



the Atlantic, though they are largely present in all except the polar 

 seas, and range in depth from 400 to 2900 fathoms. 



(2) Pte?-opod Ooze. — The thin and delicate shells of the mollus- 

 can groups, known as the pteropods and heteropods, abound at the 

 surface of the warmer parts of the ocean, but their dead shells 

 are found only in depths of less than 2000 fathoms. In shallow 

 water (and even in greater depths near land) the shells are con- 

 cealed by other kinds of material, but in moderate depths, far from 

 any land, these shells sometimes become so frequent in the fo- 

 raminiferal ooze as to give it a special character. In its typical 

 development this pteropod ooze has been found only in the Atlan- 



