MASS DYNAMIC ACTION. 303 
As excellent illustrations of the development of rocks showing all or 
many stages of the recrystallization of quartz and the development of 
mica may be cited the mica-schists and mica-gneisses which I have de- 
scribed in the Penokee and Marquette districts of Michigan and in the 
Black Hills of Dakota (plate 19, figure 2).* 
As a beautiful illustration of the transition from finely crystalline to 
coarsely crystalline rocks may be cited the iron-bearing formation of 
the Marquette district of Michigan.f The deformation of this formation 
was mainly by recrystallization. In the eastern part of the district 
granulation and wider spaced fractures occurred to some extent, but the 
temperature was not high enough for coarse crystallization, or some 
other condition was lacking. In the western part of the district, while 
the rocks were probably not more deeply buried, the deformation was 
much more profound, and because of this itis probable that the tem- 
perature reached 180° C. or more. As a consequence the mineral parti- 
cles grew toa much larger size. At places in the eastern part of the 
district, where the conditions were least favorable for recrystallization, the 
quartz granules in the jaspilite average about .01 millimeter in diameter. 
In the western part of the district, where the conditions were more 
favorable, the quartz particles in the coarsest jaspilite average about 1 mil- 
limeter in diameter. Moreover, they show little strain. These particles 
therefore average about a million times greater than those of the eastern 
part of the district, and hence to form one new individual the material 
of a million old particles was utilized. This illustration therefore gives 
conclusive evidence of the capacity of quartz to accommodate itself to 
the most intense deformation by recrystallization. 
Reerystallization lags behind deformation.—In the deep-seated zone 
adjustment may not lag far behind the disturbing forces. However, in 
most cases there is apparently some lag. In the most regularly lami- 
nated of the crystalline schists a close examination usually shows a 
slight undulatory extinction, and therefore a state of strain in the min- 
erals, showing that recrystallization has not exactly kept pace with 
deformation, or else that they have been somewhat deformed nearer the 
surface since recrystallization. 
Where such subsequent deformation has not taken place, the amount 
of strain-shadows or granulation is thought in many cases to be a | 
measure of the amount that molecular readjustment lags behind the 
disturbing movement. In the typical crystalline schists strain is in 
many cases scarcely perceptible. In other cases all of the mineral 
* Mon. xxviii, cit., pp. 448-450, 452-454, 456-459. Mon. xix, cit., pp. 305-318. The pre-Cambrian 
rocks of the Black Hills, by C. R. Van Hise: Bull. Geol. Soe. Am., vol. i, 1890, pp. 222-229. 
} Mon. xxviii, cit., pp. 381, 391. 
