272 H.C. Sargent—Carboniferous Cherts in Derbyshire. 
Davis! calls attention to the very important point that the 
radiolarian earths of Barbados are believed to represent true deep- 
sea siliceous oozes, yet there is no evidence of the solution of silica 
on the sea-floor, and these siliceous organic remains have not been 
converted into chert. He also points out that the white diatomaceous 
earths of the Monterey group in California, which must have been 
associated with much organic matter, are almost unconsolidated 
and show no evidence of solution. 
Dr. F. W. Clarke? states that the silica extracted from the ocean 
by radiolarians, diatoms, and siliceous sponges “‘is finally deposited 
on the sea-floor and does not redissolve, or at ieast not readily ’’. 
He adds: “ The insolubility of silica in sea water is great but not 
absolute.” 
SouRCcE OF THE SILICA. 
The formation of chert by direct chemical precipitation of 
inorganic silica held in solution in sea-water, which is supported 
in this paper, has found many advocates in the past. Among these 
may be mentioned Hull and Hardman,? Prestwich,* and, more 
recently, Tarr,” Van Tuyl,° and, in part, Twenhofel.’ 
‘Upwards of 319,000,000 metric tons of dissolved silica are annually 
contributed to the sea in colloidal form by the rivers of the world.® 
Yet the amount present in normal sea-water is practically nil. 
This fact clearly indicates, as Tarr? and Twenhofel” point out, 
the completeness of the precipitation. 
Within the area of the deposition of muds and fine silts the 
dissolved silica appears to be carried down and to form a large 
part of the colloidal constituent, giving plasticity to clays." Ashley 
has shown in an important paper that the colloidal matters of clay 
are “‘non-crystalline, hydrated, gelatinous, aluminum silicates, 
organic colloids, gelatinous silicic acid, and hydrated ferric oxide ”’.!2 
Dr. W. T. Gordon has kindly pointed out to me that H. C. Barton #8 
has shown the impertance of colloids, acting as an agglutinant, 
in the formation of sandbanks and deltas. He states: 
1 Op. cit., p. 370. 
* The Data of Geochemistry, 1916, p. 130. 
3 “ On the Nature and Origin of the Beds of Chert in the Upper Carboniferous 
Limestone of Ireland’’: Sci. Trans. Roy. Dub. Soc., vol. i, 1878, pp. 71-94. 
4 Geology, Chemical, Physical, and Stratigraphical, vol. ii, 1888, pp. 320-4. 
> Op. cit., p. 427 et seq. 
6 “ The Origin of Chert’’: Amer. Journ. Sci., vol. xlv, 1918, p. 456. 
7 “ The Chert of the Wreford and Foraker Limestones along the State Line 
of Kansas and Oklahoma’’: Amer. Journ. Sci., vol. xlvii, 1919, pp. 407-29. 
8 F. W. Clarke, Data of Geochemistry, 1916, pp. 115-16. 
9 Op. cit., p. 429. 
10 Op. cit., p. 417. 
11 Cf. Leith and Mead, Metamorphic Geology, 1915, p. 104. 
12 “The Colloid Matter of Clay and its Measurement’”’: U.S. Geol. Surv. 
Bull. 388, 1909, p. 12. 
13 ““ The Work of Colloids in Sandbank and Delta Formation’’: Geogr. 
Journ., vol. li, 1918, pp. 100-12. 
