262 THREE CRUISES OF THE " BLAKE." 



The specimens examined are procured by the collecting cylin- 

 ders of sounding-machines, and the results are corrected to a 

 certain degree by those obtained from the larger masses of ma- 

 terial brought up by the dredge or trawl. The bottom deposits 

 collected by the latter instruments may have had some of their 

 smaller particles washed out on their way to the surface, and 

 they are also exposed on their upward journey to diiferent phy- 

 sical conditions. This may affect their relative chemical com- 

 position, so that in examining these larger masses care should 

 be taken to consider them merely as indicating the nature of 

 the bottom deposits. Our ultimate knowledge of these de- 

 posits is derived from the most careful microscopical ' investi- 

 gation, supplemented by chemical 

 analyses of the organic and inor- 

 ganic substances which they con- 

 tain. The presence of amprphous 

 mineral matter, of calcareous shells 

 and other skeletons of inverte- 

 brates, and of minute organic par- 

 ticles, greatly complicates this ex- 

 amination. In addition, all these 

 particles have been subjected to 

 the action of many agencies, chem- 

 Fig. 177. — Spherule of Bronzite from ical and physical, the nature of 



3,500 fathoms^ Centnd Pacific. ^hich is UOt Well Uuderstood, and 



have had their original characters 

 more or less modified by these agencies. 



Marine deposits are composed of organic and inorganic par- 

 ticles, derived, on the one hand, from the rocks of the conti- 

 nental masses and oceanic islands, and, on the other, from the 

 remains of the animals and plants living in the surface waters 

 and on the floor of the ocean. There are, however, in some of 

 the deposits of very deep water far from continents certain mi- 

 nute metallic and magnetic spherules (Figs. 177, 178, 179), and 

 other small fragments, called cho?idres, which Mr. Murray has 

 shown to be of cosmic origin, — the dust of falling stars and 

 meteorites accumulating on the bed of the ocean in those places 

 where the rate of deposition is at the minimum. 



