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 different 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 amorphous 
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 
on арте ра Padi. which is not well understood, and 
noil 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 chondres, 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. 
