GEOLOGY OF BOTTOM OF SEAS—DE LAUNAY. 337 
submerged mountain range. It may be said on the subject of sedi- 
mentation, however, that on steep slopes, deposits must hold so inse- 
curely that sediments comparatively thin must have a tendency 
toward presumed horizontality. 
To limit ourselves to two principal examples, the topography of 
the Atlantic is rather simply characterized by two deep zones extend- 
ing north and south that indent a plateau surmounted by volcanic 
cones. In the Pacific, there are first of all, along the coast, some deep 
hollows corresponding with the folded ranges of the shores. But there 
is also a long series of hollows extending in an east and west direc- 
tion from China toward the Gulf of Mexico, and, in the south there 
is a north and south hollow at right angles to the preceding which 
passes to the east of New Zealand, a hollow consequently having 
a direction like that of the American continent or of the Atlantic 
Ocean. These accidents well indicate the complexity of the topog- 
raphy of this ocean, which has sometimes been inaccurately plotted, 
and they imply a very complex ensemble of folds pertaining to vari- 
ous ages, with vertical subsidences, just as on the continents. 
The horizontal aspect of the nonlittoral sediments should be con- 
sidered only as a tendency accentuated with time by the very effect 
of some of the first deposits under which the inequalities of the bottom 
have been leveled. It has sometimes been held that when the sea 
has invaded a continent it has been preceded by what has been called 
a marine abrasion. And, in fact, it is possible, when the movement 
is made slowly after a period of emergence which has caused a pene- 
plain, that this kind of tide has almost invariably displaced horizontal 
sediments. It could not have been the same wherever the vertical 
displacement was a sudden matter nor wherever the invasion of the 
sea was the result of a folding in a geosynclinal zone. If we come 
back to the present epoch, there is every evidence that strictly con- 
temporaneous sediments are being produced to-day, some at 8,000 
meters and more in depth, others but a few meters below the level of 
the waters. Let us, then, exclude the abysmal hollows, for we have 
seen that they lengthen the discussion. Certainly the level of con- 
temporaneous sediments similar to those of geologic periods may differ 
according to location by some 2,000 meters. One sees, therefore, 
how inexact it is to prejudge the general horizontality of sediments 
pertaining to the same ancient epoch. 
(6) VARIABLE NATURE OF SEDIMENTS, AND DIAGENESIS. 
What is now going on in the bottom of the ocean? I mean, what 
is going on there in the very special order of geologic ideas? The 
first thing that interests us is that sediments are being deposited there 
and that their nature differs according to the depth of the water, the 
distance from continents, the direction of currents, the temperature 
73176°—sM 1914——_22 
