ROMANCE OF DEPTHS OF THE SEA 101 



long periods of time, the thin calcareous shells of 

 the Foraminifera are soluble in sea-water, and 

 thus it comes about that sinking through a depth 

 of some 2,500 fathoms the Globigerina shells are 

 dissolved and so never reach the bottom. In 

 their place we find at depths, say below 3,000 

 fathoms, the bed of the sea covered either with 

 volcanic material, or with flinty Radiolaria skele- 

 tons, or, still more rarely, with a Diatome ooze 

 also siliceous. Like the Foraminifera, the Radio- 

 laria are unicellular organisms buoyed up by 

 floats of various kinds on the surface of the sea. 

 But instead of secreting a skeleton of carbonate 

 of lime, their skeleton is composed, like that of 

 many sponges, of spicules of flint or silica. This 

 substance is much less soluble in water than 

 chalk, and consequently radiolarian ooze covers 

 a considerable area of the sea bottom in the 

 very deep waters of both the Pacific and Indian 

 Oceans. The second siliceous deposit is distin- 

 guished by the predominance of the skeletons 

 of the lowly forms of plant life, the Diatomes. 

 This deposit is particularly associated with a 

 band across the north of the Pacific and a great 

 girdle round the Antarctic land. 



In describing this imaginary journey across the 



