128 
PKOGRESS OP MICROSCOPICAL SCIENCE. 
which has removed all the calcareous forms, leaving only the silicious 
structures to constitute the permanent stratum? I am disposed to 
adopt the latter opinion, for several reasons.' 
" After showing the resemblance between the residue left after 
treating certain substances with nitric acid, and the diatomaceous 
deposits, I proceed to say : 
" ' Such deposits, in these present conditions, stand out as anomalies 
in the existing order of oceanic phenomena, and have nothing resem- 
bling them except the local fresh-water accumulations which occur in 
various places. Between these, however, no real analogy exists. It 
must not be forgotten that the Virginian deposit can be traced for 
above two hundred miles ; and, being marine, would doubtless be 
mixed up with such marine products as were likely to occur along 
so extended a line. The only recorded instance with which I am 
acquainted, that exhibits the slightest resemblance, is furnished by 
M. Ehrenberg, in his examination of materials brought home from the 
South Pole by Dr. Hooker. Some pancake ice, obtained in lat. 78° 10', 
long. 162° W., when melted, furnished seventy-nine species of organ- 
isms, of which only four were calcareous Polythalamia, the remainder 
being all silicious. But even this example, remarkable as it is, does 
not supply us with any real parallelism. The deposits in question 
have never yet exhibited a single example of a calcareous organism.' 
" After referring to the European greensands, I continue : 
" ' Nature furnishes us with an agent quite equal to the production 
of such effects as we are at present acquainted with. This is carbonic 
acid gas in solution in water. Mr. Lyell has already availed himself 
of the instrument to account for the subtraction of calcareous matter 
from imbedded shells, as well as for some of the changes that have 
taken place in the structure and composition of stratified rocks. . . . 
It is easy to conceive that whilst these strata were in a less consoli- 
dated state than at present, they might be charged with water con- 
taining carbonic acid gas. This would act as a solvent of the organic 
atom of lime until the acid was neutralized.' .... 
"After venturing upon these conclusions in 1847, not as mere 
speculative guesses, but as the deliberate result of a long series of 
investigations carefully worked out, I need scarcely say how intense 
was the interest with which I read Dr. Wyville Thomson's observa- 
tions, which so thoroughly sustain and confirm the accuracy of mine. 
My conclusions were wholly derived from the microscopic observations 
of earths and rock specimens which I compared with the few examples 
of foraminiferous ooze with which I was then familiar. The ' Chal- 
lenger ' researches now show us how extensively the conditions 
described in my memoir have prevailed ; a fact which could not have 
been ascertained before the machinery for deep - sea exploration 
attained to its present perfection. But having arrived at them in a 
decided or definite manner when the materials for doing so were much 
more scanty than they now are, and when no one except myself and 
the late Prof. Bailey of West Point were giving much attention to 
the subject, I think I am justified in wishing the fact to be placed on 
record." 
