362 Tlie A?ncrican Geologist. December, i899 
This table represents the percentage gain and loss indi- 
cated by the transformation from the fresh to the partially de- 
composed rock. It is introduced to show the comparison be- 
tween this and a similar stage in the weathering of an adjacent 
diabase dike.* The partially decomposed material — analysis 
given in column II — was taken from one of the inner concen- 
tric (shelly) layers of a boulder which required only a moderate 
degree of pressure from the hand to break it intO' pieces. That 
a change (weathering) in the original fresh rock had taken 
place was manifested in the medium discoloration of the com- 
ponent minerals. It is only necessary to call attention to the 
change by hydration and oxidation, which has been accom- 
plished with a total loss of only 5.38 per cent, of which loss, the 
SiOs amounts to more than one-half. In the case of the similar 
stage of weathering in the adjacent dike, the change was ac- 
companied by a total loss of 7.4 per cent, in which the SiO-- 
shows the second greatest loss among the constituents. The 
greatest loss was in the MgO, and is readily explained as hav- 
ing resulted from the easily decomposable mineral olivine, in 
an olivine diabase. 
Of the individual constituents, 5.80 per cent of the SiO^, 
2.37 per cent of the AbOa, 11.50 per cent of the CaO, and 16.23 
per cent of the MgO have disappeared in the change. 
The comparison for the two similar stages in weathering 
for the two adjacent dikes, is well brought out in the table be- 
low, which gives the total loss for rock, per each constituent: 
I lit 
Si02 2.96 0.99 
AI2O3 0.55 0.00 
F2O3 ^ . 00 . 79 
CaO 1.07 o.rs 
MgO 0.80 5.47 
Total loss for rock, per cent 5 . 38 7 . 40 
The alkalis, Na^O and K2O, were not determined for this 
stage. It has been shown, however, that the total loss for the 
two is something under one per cent. 
*Loc. cit. p. 93. 
ILoc. cit. p. 93. 
