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488 TRANSACTIONS OF SECTION C. 
solvents, resulting from the allotropic modifications of the silica, with the pre- 
sence or absence of accessory bases such as iron and lime. ‘ Azo-humus-salts,’ 
resulting from the arrested oxidation of nitrogenous organic matter, the most 
common natural solvents for silica.?_ (Examples of such corrosive action sub- 
mitted.) Free humus-acids (from decay of non-nitrogenous organic matter) pro- 
bably act only upon such constituents of the flint as contain traces of bases. ‘The 
author’s demonstration of such action years ago referred to.* Flints embodied in 
pumiceous volcanic tuff etched by the direct differential action of alkaline solu- 
tions upon the allotropic modifications of the silica. 
5. Natural agencies concerned in the fracture of flint discussed : (a) differen- 
tial movement of the chalk strata, (b) percussion, (c) frost-fracture. Any 
empirical deductions from fracture experiments on flint must take into account 
the variations in lithological character, requiring laborious use of the chemicai 
balance. Reference to the recent work of Mr. F. N. Haward and Mr. Hazzel- 
dine Warren, and to the fracture effect in differential earth-movements of 
heavily-weighted gravel, schotter, scree, talus, &c.; tough quartzite pebbles 
undergoing ‘scarring’ instead of fracture, as seen in the Bunter pebble-beds of 
Budleigh Salterton and Sutton Coldfield. : 
6. Disruption of hollow flint-nodules into multiform fragments by the expan- 
sive force of freezing water, compared with the actual case of the disruption of a 
bomb-shell (walls 13 inch thick) at Wellington College (1881) by the same agency.° 
A striking instance of this submitted exhibiting strain-cracks crossing the nodule, 
from which a considerable portion had been split off in its disruption; the cortex 
having become opalescent, without iron-staining, owing to the absence of organic 
solvents, although the flint was obtained at a depth of 11 feet from an exceed- 
ingly ferruginous post-glacial deposit.*° ‘Flaking’ (as a phase of disruption) 
may result from expansion due to heat alternating with rapid cooling owing 
to powerful radiation under desert conditions, 
7. Normal flint being about as hard as quartz (—7), its fracture-surfaces may 
be scratched by grains of zircon (7.5), tourmaline (7.5), topaz (8), corundum (9), 
&c.,"* caught between two differentially moving flint-surfaces in a landslip or a 
glacier ;'” and the increased force of impact in sand-blast due to the high specific 
gravity and greater size of these minerals may account for definite hair-like 
striations concomitant with polishing. ‘Ice-action’ has been too freely invoked 
in such cases. ‘Satiny lustre’ (due to hydration) is a different phenomenon from 
polishing. 
B. The Genesis of Flint. 
Assuming, as we may, the presence of dissolved alkaline silicates, furnished 
by the decomposition of felspars within the drainage-basin, in which spongoid 
and other low forms of life lived and died in the Cretaceous sea, it would appear 
that we may look for the supply of free silica in the genesis of flint to the 
reaction upon those silicates of organic acids, furnished in the decomposition of 
the dead sarcodic material, which (when living) formed the basis of the ameebi- 
form constituents of the living sponges, &c., the siliceous spiculés of sponges, 
Radiolaria, &c., being found often enclosed in the flint ; and CO, (product of com- 
plete oxidation of organic matter) is known to readily displace SiO; from silicates 
of the alkalies in solution. Analyses of flints (Rammelsberg’’) give various slight 
percentages of accessory potash, lime, water, and carbon, the last being probably 
the carbonised decomposition product of the postulated dead organic matter. 
The concretionary building-up of silica’4 in the form of flint nodples under the 
hydrostatic pressure of still water (often simulating in outline the form of the 
7 “Organic Matter,’ &c. (supra cit.). 
8 Chem. and Phys. Studies, &c. } 
° In the severe frost of January 1881. 
10 Tn an excavation on the upper flank of the Stort Valley (1912). 
1 Bristow, Glossary of Mineralogy (Longmans); Dick, Nature, vol. xxxvi. 
Toy MG 
12 Nature, September 26, 1912. 
13 Mineralchemie (II., pp. 163, 164). 
1 Cf. Kalkowsky, Elemente der Lithologie, p. 274—‘ Héchst wahrscheinlich 
ist die organische Herstammung fur diese Kieselgesteine, a.s.w.’ 1 Bei 
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