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MOLECULAR DYNAMIC ACTION. Doe 
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magnetite in quartz, the secondary penetration of needles of actinolite 
and serpentine into this same mineral, and the absorption of biotite and 
chlorite during the growth of garnet and staurolite. 
However profound the alterations of molecular dynamic action, the 
original textures and structures of the rock may not be greatly affected. 
It may be that all of the original minerals composing the rock are com- 
pletely changed, and yet the original igneous or other textures be per- 
fectly preserved. The case is parallel to that of petrefaction of a wood 
in which no particle of the woody fiber remains, and yet the textures of 
the living tissue are almost perfectly preserved. The modifications of 
molecular dynamic action are mainly changes of substance, not changes 
of form. Thus all the textures characteristic of igneous rocks, such as 
granolitic, ophitic, porphyritic, etcetera, may be almost completely pre- 
served in a rock which has altered throughout. This is illustrated in 
the diabase dikes in the iron-bearing formation of the Penokee series of 
Michigan and Wisconsin. These dikes in the black impervious slates 
are diabases, but their continuations in the iron-bearing formation do 
not contain one vestige of any original mineral, but are ferruginous, 
hydrated, aluminium silicates, which in composition correspond very 
closely to kaolinite.* Yet the texture in the altered rock and in the 
diabase is the same. 
Indeed, not only may there be no tendency to destroy textures and 
structures which were originally present, but there may be a tendency 
to emphasize them. This emphasizing of old textures and structures 
results from the fact that solutions work along openings and surfaces of 
weakness. At any place in which water is present in greater volume 
than the average amount, or is more than usually active, there may be 
greater than average solution and deposition, and thus emphasis of the 
old texture or structure. Common cases are the emphasis of perlitic 
cracks and bedding planes. 
However, where large individuals alter to many small particles or un- 
dergo a secondary enlargement with needle-like’ terminations, or are 
altered in various other ways, the original textures may become much 
less definite than they were originally, although the process of modifica- 
tion rarely goes so far as to obliterate original textures. 
As a result of the preservation or emphasis of original textures during 
molecular dynamic metamorphism, it may happen that somewhat ex- 
tensive changes ina rock may be overlooked or ignored. ‘Those who are 
most familiar with the recent, little modified rocks are inclined to ex- 
plain the phenomena they see in the rocks being studied as original. 
* The Penokee Iron-bearing series of Michigan and Wisconsin, by R. D. Irving and C. R. Van 
Hise: Mon. U.S. Geol. Survey, no. xix, 1892, pp. 357, 358. 
XLIV—Butt. Grou. Soc. Am., Vou. 9, 1897 
