DE. WYVILLE THOMSON ON HOLTENIA. 
715 
chalk-mud. So remarkable are the resemblances between the two formations and their 
respective faunae, that for this and for other reasons which will be fully discussed else- 
where, the conviction has been forced upon my colleague Dr. Carpenter and myself, 
that the deposit of “ chalk-mud ” at present taking place in the Atlantic is continuous 
with the deposit of chalk-mud of the cretaceous period, and that the margins and the 
shallower portions only of the cretaceous sea-bed have been elevated into dry land*. 
One great difference, however, exists between the modern chalk-mud and the ancient 
chalk. The modern chalk-mud from all deep-sea soundings is full of delicate siliceous 
organisms, Diatoms, Radiolarians, and Sponges, while the chalk is almost devoid of 
diffused silica. From the circumstances under which silica occurs in the chalk, forming 
bands of flint, masses of amorphous silica which have evidently taken the form of any 
cavities or moulds occurring in the beds, we have been led to believe that the silica 
existed originally in the form of diffused organisms in the chalk of the cretaceous period 
as in the modern chalk, and that it was afterwards dissolved out ; and filtered through the 
chalk, and retained in a colloid state in the cavities, by a process of dialysis. If this view 
be correct, we should be inclined to regard the Ventriculites in their ordinary state of 
preservation, as the moulds of delicate siliceous fabrics either identical with or very 
closely allied to the recent vitreous, sponges ; from which the silica has been removed, 
its place being occupied by stained carbonate of lime. 
The habitat and the conditions of life of the Vitreous Sponges. 
The geographical distribution of the order seems to be very wide. Euplectella is found 
in the Philippine Archipelago, and Habrodictyon off the Isle of Bourbon. Hyalonema is 
abundant in the seas of J apan, and has been procured in deep water off the coast of Por- 
tugal, near the north coast of Scotland, and at Santa Cruz. Aphrocallistes Bocagei was 
procured at the Cape de Verd Islands, and JDactylocalyx is abundant in the seas of the 
Antilles, and has been brought from the Azores and from Madeira. With the exception 
of Dactylocalyx , which, so far as I am aware, has hitherto been only found scattered like 
pieces of pumice on the sea-shore, but which appears from its form to grow attached to 
something, the habitat of the whole vitreous order of sponges is oozy calcareous mud. 
Soundings over a great part of the North Atlantic, embracing the Telegraph Plateau and 
the greater part of the vast region swept by the Gulf-stream, and also over a large portion 
of the Pacific, have shown that an important calcareous deposit is taking place throughout 
the whole of the warmer regions of the ocean. This deposit, so far as it has hitherto been 
investigated, presents a very uniform character. It consists mainly of fine particles of 
carbonate of lime, and the sounding-apparatus always brings up a considerable proportion 
of the fresh shells of Globigerina with smaller numbers of the shells of other Bhizopods. 
The sounding-lead sinks several feet into this fine deposit ; and it seems that the deeper 
* “ Preliminary Report, by Dr. William B. Carpenter, Y.P.R.S., of Dredging Operations in the Seas to the 
North of the British Islands, carried on in Her Majesty’s Steam-vessel ‘Lightning,’ by Dr. Carpenter and 
Dr. Wxville Thomson,” Proceedings of the Royal Society, vol. xvii. 
