Thus, in the whole of the “intermediate zone,” the silicious deposit which is being formed there, as elsewhere, by the accumulation of sponge-spicula, Radiolaria, and Diatoms, is obscured and overpowered by the immensely greater amount of calcareous sediment, which arises from the aggregation of the skeletons of dead Foraminifera.  The similarity of the deposit, thus composed of a large percentage of carbonate of lime, and a small percentage of silex, to chalk, regarded merely as a kind of rock, which was first pointed out by Ehrenberg,[5] is now admitted on all hands; nor can it be reasonably doubted, that ordinary metamorphic agencies are competent to convert the “modern chalk” into hard limestone or even into crystalline marble.

[Footnote 5:  The following passages in Ehrenberg’s memoir on The Organisms in the Chalk which are still living (1839), are conclusive:—­

“7.  The dawning period of the existing living organic creation, if such a period is distinguishable (which is doubtful), can only be supposed to have existed on the other side of, and below, the chalk formation; and thus, either the chalk, with its widespread and thick beds, must enter into the series of newer formations; or some of the accepted four great geological periods, the quaternary, tertiary, and secondary formations, contain organisms which still live.  It is more probable, in the proportion of 3 to 1, that the transition or primary period is not different, but that it is only more difficult to examine and understand, by reason of the gradual and prolonged chemical decomposition and metamorphosis of many of its organic constituents.”

“10.  By the mass-forming Infasoria and Polythalamia, secondary are not distinguishable from tertiary formations; and, from what has been said, it is possible that, at this very day, rock masses are forming in the sea, and being raised by volcanic agencies, the constitution of which, on the whole, is altogether similar to that of the chalk.  The chalk remains distinguishable by its organic remains as a formation, but not as a kind of rock.”]

Ehrenberg appears to have taken it for granted that the Globigerinoe and other Foraminifera which are found in the deep-sea mud, live at the great depths in which their remains are found; and he supports this opinion by producing evidence that the soft parts of these organisms are preserved, and may be demonstrated by removing the calcareous matter with dilute acids.  In 1857, the evidence for and against this conclusion appeared to me to be insufficient to warrant a positive conclusion one way or the other, and I expressed myself in my report to the Admiralty on Captain Dayman’s soundings in the following terms:—­

“When we consider the immense area over which this deposit is spread, the depth at which its formation is going on, and its similarity to chalk, and still more to such rocks as the marls of Caltanisetta, the question, whence are all these organisms derived? becomes one of high scientific interest.