178 REPORT—1843, 
a depth between twenty and thirty-five fathoms. The thinness of the layer 
of organic remains resting in pumice indicated that no long period had past 
since a former disturbance of the bottom. The state of the bivalves, their 
shells double and their valves closed, with the epidermis remaining, indicated 
that they had been suddenly destroyed, for when Pectunculi and Arce die 
naturally the valves either separate or remain gaping. ‘They had, doubtless, 
been smothered in the shower of pumiceous ash which now covers them. 
The Bay of Santorin, close to the island in question, afforded us no sound- 
ings with 150 fathoms line, so that either a high bank, on which lived the 
Mollusca enumerated, existed there in 1707, before the eruption, or the bot- 
tom was uniformly such as the association of animals on it certainly indicates, 
in which case a depression of more than 100 fathoms must have taken place 
in consequence of the convulsion. 
A similar application may be made of the knowledge of associations of 
species in depth to the elucidation of the deposits of the tertiary and even of 
older periods. ‘The determination of the depth by such means is of great 
importance, for we have already seen how calculations as to climate and 
northern or southern character of fauna may mislead, unless we attain a 
knowledge of the region in which the strata were deposited. 
The bottom-of the Aigean is probably gradually shallowing. The streams 
which pour into it are thickly charged with sediment. The lowest depth 
explored was 230 fathoms. Now when the sedimentary deposit shall have 
filled up that region and brought it to the lowest range of the region next 
above, it will present a thickness of 725 feet. We have seen that this lowest 
region had everywhere a bottom of yellowish mud, and that similar animal 
forms prevailed throughout its extent. Now the strata which shall have been 
formed by the filling up of that region will present throughout an uniform 
mineral character closely resembling that of chalk, and will be found charged 
with characteristic organic remains and abounding in Foraminifera. We shall 
in fact have an antitype of the chalk. But the #gean is far deeper through 
a great portion of its extent than 230 fathoms. The depth below this point 
will doubtless be filled with a similar mineral deposit, in places perhaps several 
thousand feet in thickness. But we have seen that the diminution in the 
number of species and of individuals as we descend in this lowest region 
pointed to a not far distant zero ; therefore the greater part of this immense 
under-deposit will in all probability be altogether void of organic remains. 
When indurated it would present the appearance of a great portion of the 
immense beds of scaglia otf Apennine limestone which form such extensive 
districts in the South of Europe and West of Asia. This is supposing no 
change of level takes place during the deposition of the chalky mud. But 
any depression, rapid or gradual, will add to the extent of this great stratum, 
and by supposing such phznomenon to occur,—and the probability of its 
occurrence is attested by numerous examples of such in the Archipelago,— 
we may have a cretaceous formation produced of uniform mineral character 
and of indefinite thickness. On the other hand, any elevation, by raising the 
upper portions of the lower zone into the region next above it, will cause a 
correspondent change in its fauna, and if a depression ensue, we shall have 
an alternation of faunas, indicating very different depths and presenting very 
distinct zoological combinations. 
Similar considerations respecting the other regions in depth must occur to 
the zoo-geologist who examines the facts embodied in the catalogues and 
tables of this report. I shall not swell its pages further by entering more at 
length into this attractive portion of my subject, which I leave to the conside- 
le 
nial 
