MOLECULAR DYNAMIC ACTION. 287 
alterations in connection with such deformation the term dynamic met- 
amorphism is usually restricted. Mass dynamic action is always accom- 
panied by molecular dynamic action. It is recognized that there are all 
gradations between molecular dynamic action and mass dynamic action. 
However, in many regions the phenomena are produced in connection 
mainly with one or the other. 
MoLEcULAR DYNAMIC ACTION 
Molecular dynamic action involves various degrees of movements. 
(1) Presumably the lesser movements are the cases of change in 
crystalline form and of strain within the elastic limit. In the change 
of a substance from one crystalline form to another—as, for instance, of 
aragonite to calcite—the movement of the molecules may not involve 
more than a redistribution or rearrangement of those which are adja- 
cent. In the case of substances strained within the elastic limit the 
molecules are simply pressed slightly closer together or pulled slightly 
farther apart, and yet these very shght adjustments may have a most 
profound effect upon the physical properties of the materials. For in- 
stance, amorphous glass when strained but slightly and well within its 
elastic limit becomes an anisotropic substance. Leucite crystallizes in 
the isometric system at high temperatures. As the mineral cools it 
passes at once into an anisotropic form. ‘The alteration from one form 
to the other may be seen by alternately heating and cooling this mineral 
under the microscope. In the foregoing cases, while we cannot doubt 
that movement occurs, the readjustment is molecular, and is therefore 
beyond the power of the microscope to determine its character. 
(2) Ina second class of movements there is a rearrangement of the 
chemical elements by which new compounds are produced from old 
compounds. Material may be added to or subtracted from a given 
mineral or from glass, or either minerals or glass may be altered into 
two or more other minerals, with the simultaneous addition or subtrac- 
tion of material. The added material in any case may come from some 
other particle not far distant. The material subtracted in any given 
case may be added to another particle at a greater or less distance. 
Illustrating the above are the alterations of feldspar into muscovite and 
quartz; and olivine into serpentine and ferrite. The motions involved 
in these changes are confined within such short distances that the naked 
eye does not ordinarily discover the relations of the original and sec- 
ondary minerals. Such movements are microscopic. 
In such changes in the case of the dense rocks, in which there is little 
