194 The American Geologist. soptomher. looi. 
in acidit}- in the protogine the authors heHcve cannot ho due 
to a lack of honio,q;cneity in tlic original magma, hut are to he 
attrihuted to modifications, more or less local and partial, re- 
sulting from the action of the crystalline covering, /. e., the 
schists, upon the rock "de profondeur." Going beyond the 
massive nucleus, the authors find that the surrounding schists 
having 59.30 per cent of silica, are still less acid than the sur- 
rounding gneissic rock. In other words, that which is the most 
acid rock of the series is that which presents the strongest 
igneous characters, and which has most frequently and effect- 
ivelv penetrated the less acid in the form of hosses and dikes. 
The degree of acidity varies pari passu with the degree 
of fluidity of the rock considered. Such difference in acidity 
has frequently been appealed to to prove the genetic difference 
of the schists from the granite. There is no more obvious or 
common remark than that a crystalline rock, svich as a granite,, 
resulting perhaps from a fusion of fragmental materials, ought 
to contain substantially the same degree of acidity as the aggre- 
gate of the materials from which it was derived. T^iiere is 
nothing more evident. But it cannot be inferred from this 
that the granite at a definite point of intrusion should show the 
same degree of acidity as the schists which it intrudes, for the 
very fact of the intrusion implies that the gTanite has been 
moved from the point at which it may have originated, and 
hence that it may have started with a greater amount of acidity. 
If the action of quartz under metamorphism be considered a 
moment it will be found as a general law that the more acid 
rock material fuses sooner than the less acid. Quartz is the 
most mobile under metamorphism of all tlie rock-forming 
minerals. It is both first 'and last to show the effect of met- 
amorphism. If such metamorphism be continued till it re- 
sults in plasticity or fusion, it is an inevitable consequence that 
tht most acid material would first become plastic. Hence, in all 
cases, when not disturbed by abnormal or accidental conditions,, 
acid intrusives are found penetrating the less acid schists. 
Acid nuclei, in granitic mountain districts, are surrounded by 
less acid rocks, and inclusions of the less fusible schists are- 
found in the granitic mass. In all cases, whenever a single- 
epoch oi metamorphism is considered, the more acid intrudes 
the less acid. In the rare case of dikes more basic than the- 
