258 Mr. J. S. Bowerbank on the Siliceous Bodies of the Chalk. 



with half an inch in thickness or more of pure chalcedony, and 

 then succeeds a crop of regular crystals of quartz. The like is 

 familiar to every mineralogist in agates from rocks of volcanic 

 origin, in the cavities of which, the water, perhaps containing 

 but a very few per cent, of silex, may by gradual and continuous 

 filtration have deposited the silex long after the rocks had ceased 

 to possess a greater degree of heat than the ordinary temperature 

 of the earth. 



If, on the contrary, we imagine a high degree of thermal heat 

 necessary for the conveyance and deposit of the silex, how is it 

 that the water at this high temperature spares the carbonate of 

 lime in the beautiful and delicate shells which are often attached 

 to the surface of the chalk flints, and the numerous remains of 

 cartilaginous and other fishes, Crustacea, and other delicate animal 

 remains which abound in a most perfect condition amidst the 

 very flints that are supposed to require so great a degree of 

 thermal heat for their formation ? And if the deposit of the silex 

 be determined by any great degree of thermal heat, it may na- 

 turally be supposed that it would be deposited somewhat in the 

 form of that precipitated from the waters of the Great ^Geyser 

 and other such springs ; but this is not the case. In the flints 

 and agates the normal form of the deposit is the compressed 

 acicular crystallization of chalcedony; while in the latter it 

 is purely amorphous, the highest power of the microscope afibrd- 

 ing not the slightest indications of crystallization : in fact, it is 

 the well-known mineral, siliceous sinter. I have examined spe- 

 cimens of this mineral begCring the impressions of leaves recently 

 brought from the Great Geyser by Mr. C. C. Babington, and with 

 a power of 500 linear it presents a purely resinous or glassy 

 structure ; not the slightest trace of radiating crystallization even 

 from the parts which bear the impress of the leaves. 



The author then treats of the fossilization of Choanites and 

 Ventriculites, and describes them as imbedded in flint, and pos- 

 sessing " a light floating elegance of form as if still enjoying life 

 in their native liquid element ; and which facts assure us that 

 they were thus suddenly and instantaneously fixed in a moment 

 of the highest vitality.-'' I really cannot understand how the 

 author arrives at this conclusion, that because they retain their 

 form they were necessarily imbedded alive. We are all familiar 

 with the very long time that a piece of common sponge will do 

 duty in a water-filter, for months or even years, without the 

 destruction of its texture; and the recent genera to which the 

 fossil sponges termed Ventriculites and Choanites belong, are of 

 a much stronger and more horny structure. The recent type of 

 the former I have received from my friend Capt. Ince, R.N., who 

 procured it at Torres Straits, and another species from the Phi- 



