GEOLOGY OF BOTTOM OF SEAS—DE LAUNAY. 351 
brought up some Cretaceous blocks, showing that marine crustacea 
might have existed between the two important peninsulas of Corn- 
wall and Brittany, and consequently justifying the inference that 
since the Cretaceous epoch this might have been the site of a marine 
furrow. The same series of dredgings enables us to trace on the map, 
from Morlaix to Plymouth, after these Eocene and Cretaceous epochs, 
some successive zones of Liassic, then of Triassic, with ancient strata 
in the vicinity of the continent. Thus a series of ancient seas might 
have existed on the site of the Channel to lead, probably during the 
beginning of the Pleistocene, to an emergence during which the Chan- 
nel must have played the part of a continental valley and finally 
resulted in a last submergence of which we see the effects. 
On the Atlantic coast we have but little information in regard to 
the continental shelf or plateau to the west of Ireland, where the 
zones of strata which beeome visible on the continent appear to be 
prolonged under the sea. In the Atlantic Ocean itself volcanic rocks 
are found at various points denoting high bottoms not covered over 
with sediments. Notably, 900 kilometers north of the Azores, at a 
depth of 3,000 meters, some jagged flows of lava have been found 
which were almost certainly solidified in the air.t 
Everywhere else we are awaiting information not yet furnished us 
by oceanographic soundings, and in order to reconstruct the history 
of the ancient seas we are reduced to deductions based on the con- 
tinuity of geologic zones, on the nature of the faunas and the floras in 
various epochs, and on the relations between these faunas and floras. 
In prief, we are led thus to conceive the various types of seas: 
(1) The type of continental shelf or plateau prolonging the neigh- 
boring continents in a gentle slope, which the faint movements of 
equilibrium may alternately move again above or below the sea (the 
North Sea, the Baltic, Hudson Bay), ete.; (2) the type of geosyn- 
clinal depression in connection with foldings which take at first the 
‘aspect of a marine furrow or ridge, to give place some day to a high 
alpine chain and on which, in proportion to its fragility, become 
accumulated volcanic and seismic irregularities (Mediterranean) ; 
(3) the Atlantic type, where successive subsidences through irregu- 
larities of unequal depth extending north and south have abruptly 
sliced off geologic zones of nearly perpendicular direction, and where 
the slopes cross these perpendicular zones without the least account 
being taken of them, a recent ocean at its central axis elevated with 
volcanic manifestations;? and, finally, (4) the Pacific type in which 
geosynclinal troughs with ranges of folding warp an immense block, 
1 Pruvot and Robert studied in 1897 near Cap de Creus (Arch. zool. expér. et génér., pp. 497-510) a sub- 
marine deposit of shells anterior to the second half of the Pleistocene, at the beginning of which the present 
régime was established in the Mediterranean and the Septentrional mollusks had disappeared. 
2 See on this subject la Science géologique, p. 253. 
