Weathering of Diabase in Virginia. — Watson. 367 
A glance at the above table is sufficient to show the differ- 
ence in the physical state of the two rock products. In the 
case of the Medford rock, it is found that the three grades, 7, 
8 and 9, representing, silt, fine silt and clay, respectively, corre- 
spond to only 3.17 per cent of the whole, while in the case of 
the \'irginia rock these correspond to 42.34 per cent of the 
whole. These three grades in the above table may safely be 
taken to represent the actual decomposed portion of the rock, 
although some particles of undecomposed minerals may, at 
times, be present in grades 7 and 8. 
The clay content, which is truly decomposed material, 
amounts to 14.20 % in the Virginia rock as against 1.67% 
in the Massachusetts. In examining the table of losses 
for the entire rock, we find that the results on chemical anal- 
yses, for the same two rocks, columns I and I\', indicate a total 
loss, respectively, of 70.31 per cent, Va., and 14.93 P^i' cent, 
Mass. That is to say, the transformation from the fresh to de- 
composed rock in the Virginia locality is accompanied by a 
total loss of 70.31 per cent, w^hich is equivalent in the mechani- 
cal analyses of the residual product to 42.34 per cent; and 
similarly for Massachusetts a total loss of 14.93 P^r cent, equal- 
ing 3.17 per cent of the actual decomposed clay. 
It appears, therefore, that a certain fairly definite relation- 
ship exists between the mechanical and chemical analyses. 
That the mechanical work affirms whether or not the degenera- 
tion is wholly chemical or physical, or a combination of the 
two ; and further indicates which of the two was the most prom- 
inent in breaking down the fresh rock. 
The table shows that the degeneration in the Virginia rock 
has been principally due to chemical work, while that of the 
Massachusetts is due principally to the work of disintegration 
(physical forces) aided by decomposition (chemical forces). 
Concerning the degeneration of the diabase rock at Medford, 
Mass., Dr. Merrill* says: 'Tn this case, as in that of the gran- 
ite from the District of Columbia, we have to do with only the 
earlier stages of degeneration, with conditions which areas 
much in the nature of mechanical disintegration asof chemi- 
cal decomposition." 
*Merrill, G. P., Rocks, Rock-Weathering and Soils, N. Y. 1897, p. 
222. 
