288 VAN HISE—-METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
mass dynamic action, chemical analyses appear to show that the aver- 
age composition of the rocks does not greatly change, except by hydra- 
tion. However, there may be interchange on a great scale between the 
minerals within short distances, and glasses may wholly devitrify, but 
the migrations of material are ordinarily confined within somewhat 
narrow limits. 
The profound changes produced in connection with molecular motions 
in the dense rocks are largely accomplished by water transporting the 
compound in solution. The spaces between the particles, even those of 
subeapillary size, may be penetrated by solutions and the unstable min- 
erals transformed throughout. But under mass static conditions, where _ 
the spaces bearing water are very minute, the changes are exceedingly 
slow. Even in the pre-Cambrian rocks, in the larger dense masses, such 
readily alterable minerals as nephelene and olivine are found. Olivine 
occurs extensively in the pre-Cambrian (Keweenawan) diabases and gab- 
bros of the Lake Superior region. Throughout extensive masses in this 
region the less alterable minerals, augite and basic feldspar, are appar- 
ently almost perfectly fresh. ‘The small masses of dense rocks, and es- 
pecially those which are in the midst of porous rocks, are much more 
altered. 
In the much fractured rocks and in the porous rocks such as sand- 
stones and extrusive igneous rocks and especially the vesicular lavas 
and porous tuffs, where there is comparatively rapid circulation of water, 
the migration of materials may be important, the addition of material 
may be large, and the alterations extend throughout the rock masses. In 
the case of the amygdaloids the added material has oftentimes been suffi- 
cient to completely fill the spaces. Moreover, throughout extensive areas 
important formations are so altered that no original minerals remain. 
In rocks altered under conditions of molecular dynamic action all 
stages of alteration may be seen, from the comparatively fresh rocks, in 
which the changes are incipient with the minerals most readily altered, 
to those rocks in which all alterable minerals have been transformed by 
metasomatism into others which are permanent under the prevailing 
conditions. 
In the majority of changes by molecular dynamic action, under both 
(1) and (2), which come within our observation, the chemical reactions 
usually result in a liberation of heat or running down of energy, under the 
first part of van’t Hoff’s law (see page 277). This may be illustrated by 
hydration, which is, perhaps, the most characteristic change of molecular 
action, such minerals as chlorite, kaolinite, zeolites, and epidote forming 
abundantly. The reverse chemical action, that taking place with the ab- 
sorption of heat, occurs only at great depth under mass static conditions. 
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