
=_ 
: 
” 
=< ‘ ~_ = w ‘ . we 
- Parr Il. Suor. ii. § 1] WEATHERING. 333 
millimetre, so that a crag of such limestone would be lowered 1 metre 
in 72,000 years by the solvent action of rain.” 
Not only carbonates but silicates of lime, potash, and soda, 
combinations existing abundantly as constituents of rocks, are 
attacked by rain-water ; their silica is liberated and partly dissolved, 
while their alkalies or alkaline earths, becoming carbonates, are 
removed in solution. The felspars for example are thus decomposed, 
the alkalies and the lime being gradually abstracted together with 
a portion of the silica. The result is a slow disintegration of the 
stone into sand and clay. 
4. Hydration—Some anhydrous minerals, when exposed to the 
action of the atmosphere, absorb water (become hydrous), and may 
then be more prone to further change. Anhydrite becomes by 
addition of water, gypsum, the change being accompanied by an 
increase of bulk. It has been suggested that local uplifts of the 
eround may sometimes have been caused by the hydration of 
large subterranean beds of anhydrite. Many substances on oxidizing 
likewise become hydrous, ‘The oxidation of ferrous oxide in damp 
air gives rise to hydrous ferric oxide, with its characteristic yellow 
and brown colours on weathered surfaces. 
Weathering.—This term expresses the general result of all . 
kinds of meteoric action upon the superficial parts of rocks. As 
these changes almost invariably lead to disintegration of the surface, 
the word weathering has come to be naturally associated in the 
mind with a loosened crumbling condition of stone. But the 
influence of the atmospheric agents is not invariably to destroy the 
coherence of the integral particles of rocks. In some cases stones 
harden on exposure. Certain sandy rocks, for example, like the 
“orey weathers” and scattered Tertiary blocks in the Ardennes, 
become under meteoric influence a kind of lustrous quartzite. In 
other cases there may be more complex molecular rearrangements, 
such as those remarkable transformations to which Brewster first called 
attention in the case of artificial glass.2. He showed that in thin 
films of decomposed glass obtained from Nineveh and other ancient 
sites, concentric agate-like rings of devitrification are formed round 
isolated poimts, closely analogous to those above described as 
artificially produced by the action of heated alkaline waters (p. 301), 
and that groups of crystals or crystallites, “ probably of silex,” are 
developed from many independent points in the decomposing layer. 
Coloured films indicative of incipient decomposition have been ob- 
served on surfaces of glass exposed only to the air of the atmosphere 
for twenty or thirty years. Brilliantly iridescent films have been 
produced on the glass of windows exposed for not more than twenty 
years to the air and ammoniacal vapours of a stable*® That 
1 Pfaff, Z. Deutsch. Geol. Ges. xxiv. p. 405, and “Allgemeine Geologie als exacte 
Wissenschaft,” p. 317. Roth, Chem. Geol. i. p. 70. Geikie, Pioc. Roy. Soc. Edin. x. 
1879-80, p. 518. 
2 Trans. Roy. Soc. Edin., xxii. 607, xxiii. 193. 
’ This fact has been observed by my friend Mr. P. Dudgeon of Cargen in an ill- 
ventilated cow-house, and I have seen the plates of glass removed from the windows. 
