338 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
of surface waters, ete. Starting with our geologic strata, we shall 
thus be permitted to discover under what conditions analogous sedi- 
ments have formerly been deposited. For the exploration of oceanic 
deposits, we are provided with well-known instruments called the 
sounding dredge or the Buchanan tube. 
In the formation of these sediments a first important demarcation 
must be established. Some are the product of the mechanical 
destruction of continents. These are clastic débris removed from 
the rocks and the soils of parts emerged, and after having been rolled 
around for a time, and for a longer or shorter period held in suspen- 
sion, they are at last deposited. These are terrigenous deposits. 
Others have begun by being in solution, but all have a hke origin, 
and they separate from these solutions, either by the medium of organ- 
isms, or by a simple chemical reaction. Finally, other chemical 
reactions, characterized as diagenesis, continue in the sediments after 
their deposition. 
The terrigenous deposits proper form a crown or belt around 
the continents. These cease at a distance from the continents and 
-are followed by sediments entirely different in character, either 
those of organic origin, resembling a grayish ooze, which we shall 
presently distinguish, or those of chemical origm which may take 
on the appearance of red clays. This distinction of terrigenous de- 
posits is very important, although one must not attribute to them, 
as is sometimes done, a too absolute value, for, correctly speaking, 
there are hardly any sediments which do not include some terrige- 
nous elements. Far distant from the coasts, however, these ter- 
rigenous elements are generally reduced to very fine particles which 
remain in suspension much longer than is generally believed and which 
contribute toward the clay of the deep ooze, at the same time that 
this clay, redissolved, as we shall see, yields the silica of siliceous 
organisms and consequently contributes to the formation of silex in 
the strata. When instead of confining oneself to a summary state- 
ment, an examination is made of certain sections of samples from very 
deep soundings, one often notes these irregular intercalations of 
extremely fine, true sandy beds? which indicate a dragging along of 
these particles by the waters to very considerable distances. 
These detrital sediments, as I have just said, come from continents; 
they are the direct product of erosion. Now, this erosion is impor- 
tant. It is estimated that under present conditions it would destroy 
the land in 7,000,000 years; each year the ocean receives 10 cubic 
kilometers of solid material, composed in great part of silica and 
alumina.” 
1 Expedition of the ‘‘ Gauss”? and works of Thoulet. 
2 The average composition of the terrestrial crust in round numbers is 60 per cent silica, 15 per cent alu- 
mina, 6 per cent iron, 5 per cent lime, 5 per cent alkalies. (See Science géologique, p. 654.) 
