MASS DYNAMIC ACTION. 309 
Therefore these slates and shales contain in combination not only a suffi- 
cient amount of water to satisfy the requirements as to combined water 
in the completely metamorphosed schists, but they contain an excess of 
water which is probably steadily given off during the process of meta- 
morphism, and thus they constantly furnish a sufficient supply of the 
medium through which recrystallization can take place. 
The slates and shales themselves are rocks partly metamorphosed by 
mass dynamic action, and they doubtless lost combined water in the 
process of metamorphism, just as additional water would be lost in their 
further metamorphism to schists or gneisses. The original mudstones 
from which the shales and slates were produced may be presumed to have 
contained at least as much water as soils and clays. Sixteen analyses of 
soils and clays from Pennsylvania, Florida, and Colorado gave an aver- 
age loss of water at or above 100° C. of 7.15 per cent. Six analyses of 
clays and soils from Virginia gave an average loss of water above 110° C. 
of 8.61 per cent. Forty-four analyses of clays and soils from Massachu- 
satts, South Carolina, Alabama, Missouri, Colorado, Nevada, and Cali- 
fornia gave an average loss of water upon ignition of 8.10 per cent.* 
If the analyses quoted on the previous pages were taken as typical of 
the finer grained sedimentary rocks, it would be concluded that in the 
metamorphism of mudstones to shales and slates about one-half of the 
combined water is lost, and that in the further metamorphism of the 
shales and slates to the mica-schists and mica-gneisses one-half of this 
residual water content is lost. 
These facts give further support to the argument for the necessity of 
water at the beginning of the dynamic process in order that recrystalliza- 
tion shall readily occur. If the massive original rocks at the beginning 
of the mass movement did not contain sufficient amounts of water so that 
recrystallization could take place, it does not appear that they are likely 
to gain sufficient water from an outside source, and hence the frequent 
granulation of the plutonic rocks when deformed. However in propor- 
tion as the rocks are deep seated, a less amount of water probably suffices 
for the process of recrystallization. 
Though aside from the present discussion, the steadily lessening 
amount of combined water with increasing metamorphism is illustrative 
of the fact, already explained (see pages 280, 281), that the deep-seated 
zone of recrystallization, as a result of dynamic metamorphism, is one 
of dehydration. 
In opposition to this, the alterations under mass static conditions, and 
especially under conditions of small depth, are those of hydration. Many 
* Loe. cit., pp. 287-301. 
XLVI—Butt. Grou. Soc. Am., Vou. 9, 1897. 
