of Edinburgh, Session 519 
The transition between the calcareous formations and the 
argillaceous ones takes place by almost insensible degrees. The 
thinner and more delicate shells disappear first. The thicker and 
larger shells lose little by little the sharpness of their contour, and 
apj3ear to undergo a profound alteration. They assume a brownish 
colour, and break up in proportion as the calcareous constituent 
disappears. The red clay predominates more and more as the 
calcareous element diminishes in the deposit. 
If we now recollect that the most important elements of the 
organic deposits have descended from the superficial waters, and that 
the variations in contour of the bottom of the sea cannot of them- 
selves prevent the debris of animals and plants from accumulating 
upon the bottom, their absence in the red clay areas can only be 
explained by a decomposition, under the action of a cause which 
we must seek to discoyen 
Pteropod ooze, ifc will be remembered, is a calcareous organic 
deposit, in which the remains of Pteropods and other pelagic 
Mollusca are j)resent, though they do not always form a preponder’^ 
ating constituent, and it has been found that their presence is in 
correlation with the bathymetrical distribution. 
In studying the nature of the calcareous elements Wliicll ate 
deposited in the pelagic areas, it has been noticed that, like the 
shells of the Foraminifera, those of the Thecosomatotis Pteropoda, 
which live everywhere in the superficial waters, especially in the 
tropics, become fewer in number as the depth from which the sedi- 
ments are derived increases. We have just observed that the shells of 
Foraminifera disappear gradually as we' descend alo’ng a series of 
soundings from a point Where the Globigerina ooze has abtindance of 
carbonate of lime, towards deeper regions j but We notice also that 
when the sounding-rod brings up a graduated series of sediments from 
a declivity descending into deep \Vater, among the calcareous shells 
those of the Pteropods and Fleteropods disappear first in proportion 
as the depth increases. At depths less than 1 400 fathoms in the tropics 
a Pteropod ooze is found with abundant remains of Heteropods and 
Pteropods ; deeper soundings then giVe a Globigerina ooze without 
these molhtscan remains; and in still greater depths, as before 
mentioned, there is a red clay in Which calcareous organisms are 
nearly, if not quite, absent. 
