354 THE MEDALS OF CREATION. Cuar.X. 
may be admitted to a great extent, yet the deposition of 
silex and lime from aqueous solutions, is carried on at the 
present moment upon an enormous scale ; and it cannot be 
doubted that to such a process is attributable the formation | 
of the nodules, layers, dikes, and veins of flint, which traverse 
the chalk, and other rocks.* | 
The most abundant microscopic organisms in the English 
chalk and flint which I have examined, are Rotalie, or 
Rosaline, and Teatularie. Immense numbers of minuter 
Foraminifera also occur, and many shells, which are unques- 
tionably the young state of testaceous Cephalopoda (as 
Nautilus, Ammonite, &c.). 
Spines of Sponges, and of Echinoderms, also frequently 
appear in the field of the microscope : and a spongeous 
structure is so common in flint, that an eminent observer 
conceives that all the flints, both nodular and tabular, have 
originated from poriferous zoophytes ;F an hypothesis alto- 
gether inadmissible. 
The assertion that the chalk every where consists almost 
wholly of organic bodies must likewise be accepted with 
some limitation. The assiduous observer who searches for 
hours chalk and flint carefully prepared, and with the aid of 
an excellent microscope, though he will meet with immense 
numbers of organisms, will often find a great proportion of 
atoms without traces of structure. Neither is there much 
variety in the easily recognizable forms of the English chalk 
(I write from my own limited experience) ; many of the 
species described by M. Ehrenberg, and others, are few and 
far between ; and I have not detected a single example of 
diatomacee. The student therefore must not be discou- 
* See my “ Memoir on a Microscopical Examination of Chalk and 
Flint,” Annals of Nat. Hist., Aug. 1845. 
+ “Memoir on the Siliceous Bodies in the Chalk, Greensands, and 
Oolite,” by J. S. Bowerbank, Esq. F.R.S. &e. Geol. Trans. vol. vi. 
p. 181. ! 
