H. C. Sargent—-Carboniferous Cherts in Derbyshire. 278 
** Professor HE. W. Hilgard found, at the mouth of the Mississippi, 
a 15 feet layer of colloidal jelly on the ocean side of the bar. He 
ascribes its formation to the precipitation of the fine colloidal 
particles by the mixing of the river waters with the salt water” 
(Pop. Sci. Monthly, March, 1912). I bave been unable to obtain 
- access to Professor Hilgard’s paper. 
The effectiveness of the ions of bivalent elements in causing 
ficeculation has been shown to be much greater than that of 
univalent ions, and it has been conclusively proved experimentally 
that colloidal silica is rapidly flocculated and precipitated by 
calcium carbonate in presence of carbon dioxide.t 
In asea in which deposition of calcium carbonate was continuously 
taking place simultaneously with the products of land-erosion, as was 
the case when the impure chert-bearing limestones of Derbyshire 
were laid down, there can be little doubt that dissolved silica would 
be contributed to the sea, and that it would be precipitated near the 
land when a sufficient concentration had taken place. The tendency 
of silica to form supersaturated solutions is an important point. 
The purer white, or pale, lower limestone, in which chert occurs 
sparingly and sporadically, was probably formed in a clear sea, 
not necessarily far from land, but under conditions when the 
contribution of dissolved Silica, or other terrigenous material, was 
exceptional and not the rule. 
The liberation of silica by carbonation of silicates would be greatly 
increased, as was pointed out by Prestwich,? and more recently by 
Van Hise,® if the area undergoing denudation consisted of basic 
igneous rocks. The abrupt transition in many parts of Derbyshire 
from the pale or white limestone to the overlying, impure, chert- 
bearing rock is very noteworthy, and would, of itself, suggest that 
movement, resulting in an increase of terrigenous deposits, had taken 
place. This view is strongly supported by the presence, as observed 
in the field, of small unconformities and thrusts, and occasional 
current-bedding.* 
The toadstones of Derbyshire often underlie the main chert- 
bearing rock at no great distance, and itissuggestedas very probable 
that some, perhaps a large part, of the sea-floor covered by these 
submarine flows was raised above sea-level by disturbance, and was 
thus brought within the range of atmospheric and chemical erosion. 
There seems, therefore, to be fair evidence that, during the deposition 
1 Of. Dean, ‘‘ The Formation of Missouri Chert’’: Amer. Journ. Sci., vol. xlv, 
1918, p. 413. Also Tarr, op. cit., pp. 434-6. 
20 Opa citu para: 
3 “‘ A Treatise on Metamorphism’”’: U.S. Geol. Surv. Mon. 47, 1904, p. 824. 
See also Van Hise and Leith, ‘‘ The Geology of the Lake Superior Region”: 
U.S. Geol. Surv. Mon. 52, p. 505. 
4 Cf. T. F. Sibly, op. cit., p. 63 and fig. 5 (p. 62). Also C. B. Wedd, “‘ The 
Geology of the Northern Part of the Derbyshire Coalfield’: Mem. Geol. Surv., 
p. 35, 1913. The author has noted unconformities in other parts of the 
limestone massif. 
VOL. LVIII.—NO. VI. 18 
