350 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
3. GEOLOGIC MAP OF THE BOTTOM OF THE SEAS. 
I come finally to the last problem, which, in spite of its great 
interest, will not detain me long, for its solution is yet hardly out- 
lined—to prepare a geologic map of the bottom of the seas, and, by 
means of that, to reconstruct the history of the oceans as one may do 
for the continents. 
What I have already said on the present accumulation of marine 
sediments explains how a Buchanan sounding tube lowered to the 
bottom of the sea does not as a rule penetrate beyond the recently 
formed ooze and consequently gives no information on the geologic 
substratum. 
The exceptions are few. Let us limit ourselves to the mention of 
some excavations whose passages have penetrated a slight distance 
under the seas—for example, those in Brittany. The only place 
where work of this kind has been done methodically is at Pas-de- 
Calais, the geologic map of which has been very nearly completed in 
view of the proposed tunnel.t But the Pas-de-Calais, for an ocean- 
ographer or a geologist, is hardly a sea. Its depth is so slight (a 
maximum of 60 meters between Dover and Sangatte) that it must be 
likened to a submarine valley across which the sedimentary forma- 
tions are prolonged very perceptibly from one shore to the other.? 
At the begmning of the Pleistocene epoch communication between 
England and the Continent could still be had on foot. That is why 
this region offered a good field for soundings which in 1875 and 
1876 gave us more than 3,000 profitable throws of the sounding lead. 
Thanks to these data a complete map could be drawn, showing the 
bendings of successive strata from the Senonian in the north to 
beyond the lower Cretaceous in the south. These soundings have 
shown that under the fissured and permeable Cenomanian and Seno- 
nian chalks there existed in the Cenomanian some impermeable 
argillaceous beds. On the condition that the tunnel be kept in this 
Cenomanian bed, or 130 meters below the sea at the lowest point, it 
could be easily constructed. You know that this project has recently 
become again a live topic. 
Aside from this very special place, we have very few items to 
glean.’ However, at various points on the Channel, at Berck, at 
St. Aubin (Calvados), again at Roscoff, there have been found some 
Kocene fossils, especially nummulites, proving that at the beginning 
of the Tertiary epoch the sea had already passed the length of the 
French coast and had there left some deposits. Farther on, toward 
England, off Plymouth, some dredgings made in the open sea have 
1See La Nature, Apr. 21, 1906, and Revue des Deux-Mondes, October, 1913. 
2 The currents which sweep the bottom prevent the present sediments from being deposited there and 
facilitate the work of the geologist. 
8 See a work by Paul Lemoine in les Annales de Geographie of November, 1912. 
