1897] SOME NEW BOOKS 419 
the original rock was soluble in hydrochloric acid and sodium car- 
bonate, the proportion is now nearly 70 per cent. The instance is 
that of a gneiss from Virginia. 
FRESH GNEISS. DECOMPOSED GNEISS. CALCULATED AMOUNTS SAVED AND LOST. 
SSS SiS = SS oe 
Percentage of Percentage of 
Bulk Soluble Bulk Soluble each con- eaeh con- 
analysis. portion. analysis. portion, Total loss. ‘stituent stituent 
saved. lost. 
$10, . 60°69 10°09 45°31 17°69 31°90 47°55 52°45 
AlzO3 . 16°89 13°54 26°55 24°86 0:00 100°00 0°00 
FezO3 . 9°06 12°88 11°80 1°30 85°65 14°35 
CHO 4:44 1°64 Abe 0°06 4°44 0:00 100-00 
MgO . 1°06 0°89 0°40 0°37 0°80 25°30 74°70 
KO) ~. 4°25 2°40 1°10 0°75 3°55 16°48 83°52 
Na,O . 2°82 1°10 0°22 0°25 2°68 4:97 95°03 
P.O; . 0°25 0°23 0°47 = 0°00! 100° 0°00! 
Loss on 
ignition 0°62 0°62 18°75 13°40 0:00! 100° 0:00! 
100-08 30°47 99°98 69°18 44°67 
1 Gain. 
This table makes at once obvious the great loss in silica, lime, and 
alkalies, and the gain in water; while it further indicates from the 
increase in the soluble portion that the state of combination in many 
of the compounds has changed. Mechanical separation and micro- 
scopic examination of the decomposed material throws further light 
on the character of the decomposition and nature of the surviving 
minerals. 
In basic rocks the loss of silica is rather less, and that of iron and 
magnesia more conspicuous, while in a French basalt the felspars 
were the first to yield, and the augite and olivine the last. 
Under the heading “ Physical Manifestations” are treated: Dis- 
integration without decomposition, influences of crystalline and rock- 
structure, mineral composition (roughening and crusting), indura- 
tion and colour changes. Original characters are sometimes lost 
entirely through weathering, ‘basalts pass into apparent argil- 
laceous deposits, and granites and gneisses with their veins and 
every structural detail well preserved, may become so soft that a 
stick can be thrust deeply into them. Chemical compounds become 
on the whole simplified by weathering, but oxidation and all chemical 
change, except hydration, seems to cease below the permanent water- 
level. More refractory and dense residues, like xenotime, monazite, 
tourmalines, rutiles and precious gems, have tended to accumulate in 
favoured spots, on account of the weathering and destruction of the 
rocks in which they were originally contained. 
The rate of weathering is influenced by composition, texture, and 
position of rocks, and by humidity of climate. Weathering in humid 
climates differs in kind as well as degree from that effected in dry 
climates as shown in California by the author and by Judd in the 
Nile delta. Prehistoric implements and old surfaces covered uncon- 
formably by newer deposits may sometimes enable us to guess at the 
amount of work done in a given time, but the ascertaining of the actual 
rate of work is difficult, and the results not very satisfactory. Geikie’s 
results, on this branch of the subject, not quoted here by the author, 
