410 THE DEPTHS OE THE SEA. [ohap. ix. 



sisted mainly of a compact ' mortar,' of a bluish 

 colour, passing into a thin — evidently superficial — 

 layer, nmch softer and more creamy in consistence, 

 and of a yellowish colour. Under the microscope the 

 surface-layer was found to consist chiefly of entire 

 shells of Globigerina bulloldes (Fig. 2, p. 22), large 

 and small, and fragments of such shells mixed 

 with a quantity of amorphous calcareous matter in 

 fine particles, a little fine sand, and many spicules, 

 portions of spicules, and shells of lladiolaria, a few 

 spicules of sponges, and a few frustules of diatoms. 

 Below the surface-layer the sediment becomes 

 gradually more compact, and a slight grey colour, 

 due probably to the decomposing organic matter, 

 becomes more pronounced, while perfect shells of 

 globigerina almost entirely disappear, fragments be- 

 come smaller, and calcareous mud, structureless and in 

 a fine state of division, is in greatly preponderating 

 proportion. One can have no doubt, on examining 

 this sediment, that it is formed in the main by the 

 accumulation and disintegration of the shells of 

 globigerina — the shells fresh, whole, and living in 

 the surface-layer of the deposit, and in the lower 

 layers dead, and gradually crumbling down by the 

 decomposition of their organic cement, and by the 

 pressure of the layers above — an animal formation 

 in fact being formed very much in the same way as in 

 the accumulation of vegetable matter in a peat bog, 

 by life and growth above, and death, retarded de- 

 composition, and compression beneath. 



In this dredging, as in most others in the bed 

 of the Atlantic, there was evidence of a considerable 

 quantity of soft gelatinous organic matter, enough 



