1086 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION 



the more recent explorations of the deep-sea with the dredge ; which, 

 bringing up half a ton of this deposit at once, has shown that it is 

 not a mere surface-film, but an enormous mass whose thickness cannot 

 be even guessed at. ' Under the microscope,' says Professor Wyville 

 Thomson l of a sample of 1^ cwt. obtained by the dredge from a depth 

 of nearly three miles, ' the surface-layer w r as found to consist chiefly 



FIG. 810. Microscopic organisms in Levant mud: A, C, D, silicious 

 spicules of Tethya ; B, H, spicules of Geodia ; E, calcareous spicule of 

 Grantia.] F, G, M, O, portions of calcareous skeleton of EcMnodermata ; 

 I, calcareous spicule of Gorgonia ; K, L, N, silicious spicules of sponges ; 

 P, portion of prismatic layer of shell of Pinna. 



of entire shells of Globigerina bulloides, large and small, and of frag- 

 ments 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 Radiolaria, a few spicules of sponges, and a 

 few frustules of diatoms. Below the surface-layer the sediment be- 

 comes gradually more compact, and a slight grey colour, due probably 



i The Depths of the Sea, p. 410. See also Voyage of Challenger, ch. in., and 

 Challenger Reports, especially Deep Sea Deposits (Murray and Benard.) 



