Acidic Eruptives of Northeastern Maryland. — Keyes. 43 
have proved that zonal feldspars, as a rule, grow more basic 
towards the center and that there is sometimes a recurrence 
of a more basic zone within the more acid layers, and remem- 
bering that the plagioclase feldspars in the Rowlandville 
granites are made up of different mixtures of the albite and 
anorthite molecules, the formation of the epidote may be at- 
tributed to a particular chemical composition of a portion of 
the feldspar which has been brought under favorable physical 
conditions. It may thus be concluded that under certain con- 
ditions and v/ith certain combinations of the albite and an- 
orthite molecules there was a special tendency towards epi- 
dotization, when the rock underwent metamorphic changes 
arising from great pressure. 
It is an excellent illustration of one of those nicely balanced 
or delicately poised cases which are met with occasionally in 
the petrographical study of rocks which have been influenced 
by crustal movements, and of a change which is to be expected 
in a region which perhaps represents a part of the denuded 
base of an old mountain range. It is but the expression of 
the universal law that in stony aggregates the whole mineral- 
ogical composition and structure are being modified contin- 
ually; in some places slowly, and others more rapidly accord- 
ing to the attendant circumstances. The ever changing 
physical conditions invariably set up continuous molecular 
shiftings in every rock, whatever may be its composition or 
its relations. Of recent years it has come to be more and more 
clearly understood that the changes undergone by rock masses 
have been occasioned by the natural tendencies of minerals to 
assume combinations more stable from those less stable. 
Wadsworth* in particular has emphasized this point. But 
the statement has not carried with them its full import and 
meaning. For, in any particular case while there is an at- 
tempt towards adjustment to satisfy a certain set of condi- 
tions, the conditions themselves continually change, sometimes 
in one direction, sometimes in another. In the production of 
these alterations in rock-masses time does not enter necessa- 
rily as a factor, although ordinarily the older a rock is the 
greater is the chance for disguising its primitive character. 
Thus in attempting to determine the original condition under 
*Nature, vol. xxxv, p. 417, 1887. 
