96 The American Geologist. August, i89» 
on the grounds that the change in the rock has been essentiallv 
chemical. Since no free quartz is present in the rock, the 
sihca has been wholly derived from the silicate minerals, 
which as fast as liberated was, in great part, dissolved by the 
water present and removed. The proportions of lime and 
magnesia removed are about equal and are greater fir these 
two than for any other constituents in the rock. As is most 
generally the case, an increased loss in the soda over that in 
the potash is shown. The marked increase noted in the water 
is what would be expected. 
Table C compares the fresh with the decomposed rock. 
Here, again the analyses show a total loss of 70.31 per cent, 
or nearly two-thirds of the original material. The loss in- 
cludes 73.64 per cent of the silica, 68.19 per cent of the alu- 
mina, 98.68 per cent of the lime, 98.81 per cent of the magnesia, 
82.46 per cent of the soda, and 77.31 per cent of the potash. 
Such a large percentage loss for the entire rock is very un- 
usual for silicious crystallines, as professor Merrill* has 
pointed out from all available analyses that this loss rarely 
amounts to more than 50 per cent of the total rock mass. In 
the majority of cases, however, the decomposed product 
analyzed, had not reached, or even closely approximated to 
its limit in chemical decay, for which some allowance and 
recognition must be made. 
A notable example is cited by professor Merrillt in the 
analyses of fresh and decomposed basalt from Crouzet, in the 
Haute Loire, France, where the total loss nearly equals that 
shown here, but dififering essentially in an unusual loss in 
the iron while the alumina remains constant, which is just 
reversed for the same two constituents in the Virginia rocks. 
The excessive loss in silica has been explained in the dis- 
cussion under table B. Of all the constituents, the magnesia 
has been removed in the largest quantity. The removal of this 
constituent finds a ready explanation in the fact that it is 
mainly derived from the olivine. The weathering of olivine 
in the presence of water, oxygen, and carbonic acid would 
naturally result in the formation of carbonates, silica, and the 
*See, Treatise on "Rocks, Rock-Weathering and Soils, 1897, p. 
324. 
flbid, p. 223. 
