GEOLOGY OF BOTTOM OF SEAS—DE LAUNAY. 839 
These detrital materials are subject to mechanical action which is 
especially visible along our shores and which contributes to the for- 
mation of littoral strata so abundant in our geologic periods. At the 
same time the river waters carry along to the sea chemical products 
of every nature which are going to meet in this general outlet and 
which gradually increases not only the salinity but also the content 
of various chemical substances, reserves on which we shall see the 
organisms drawing. 
Terrigenous muds are divided into (1) blue muds charged with 
sulphate of iron and permeated with ammonia salts; (2) red muds in 
which the iron is a peroxide; (3) green muds in which the iron is in 
the state of silicate (glauconite). These last skirt the length of the 
coasts. The facts under discussion are so well known that it is 
enough to recall their nature and [ shall limit myself to some brief 
ideas on the distribution of organisms in marine deposits, a very 
important but also very common phase of my subject.! 
Marine beds, you know, comprise a primary grand division called 
the continental plateau, terminated by the line of 200 meters depth 
and characterized by the penetration of lumimous rays under such 
conditions that plants can live there, permitting the existence of 
herbivorous animals. Certain seas, like the North Sea or the Channel, 
belong entirely to this continental plateau. Two principal zones are 
here easily distinguished, the littoral and the sublittoral, which are 
subject to the play of the tides, and where the individuals, of a limited 
number of species, are very abundant. The summit of the littoral 
zone is characterized by the level of the Balanes; then deepening, 
there are the Mytilus, the Littorina, the Patella; and finally, in the 
zone exposed only at the low tides of the equinox, the Haliotis and 
the Pecten. After these come next, from the level of low tide down 
to about 27 meters, the zone of the Laminaria with the oyster beds, 
the cuttle-fish, and the calamarians. Let us note, incidentally, that 
at this level the alge help to fix and to isolate two constituent 
elements of sea water, iodine and bromine, which are extracted 
therefrom. 
Lower down, from 27 to 92 meters, we have a zone which comprises 
the important fishery regions frequented by the cod, plaice, tur- 
bot, and sole. Finally we come to the continental plateau, where, 
under very special conditions, are found the coral formations, which 
play a part so important in the formation of geologic strata. As 
regards coral the most important thing for us to remember is that 
coral organisms live only in very pure water where the surface tem- 
perature does not fall below 20° and where the variation does not 
exceed 6°; in fact, at a depth which, according to recent measure- 
1 See a very good résumé of this question in Collet’s “Les fonds marins.” 
