292. VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
It follows from the foregoing that in metasomatic changes which occur 
in rocks there is no necessity for the rapid circulation of underground 
waters, or even for any circulation beyond that necessary to provide 
water for the hydrated minerals formed by the alterations and to keep 
intact the minute amount of water in the subcapillary spaces where the 
attraction extends from wall to wall. If these conditions obtain at a 
moderate temperature, a very small amount of water may be the medium 
through which the rocks may be completely altered and recrystallized. 
DISTINCTIVE FEATURES OF MOLECULAR DYNAMIC ACTION ARE GROWTH OF 
LARGE INDIVIDUALS AND PRESERVATION OF TEXTURES 
The most distinctive features of molecular dynamic action are the growth 
of large mineral individuals and the preservation or emphasis of original 
textures and structures. 
The formation of large individuals is a result of the physico-chemical 
law explained on pages 274, 275, under which large individuals form at the 
expense of smaller ones. The exceptional growth of certain individuals 
may be recognized in the very unequal size of the mineral particles, in 
the enlargement of old individuals, and in the development of porphy- 
ritic constituents. The general unevenness of crystal particles in rocks 
altered under static conditions is so well known that the point need not 
be emphasized. The enlargement of mineral particles under mass static 
conditions has been described as occurring in quartz, feldspar, horn- 
blende, augite, garnet, tourmaline, and other minerals, by Sorby, 
Irving, Becke, Williams, Hobbs, Whittle, myself, and others. As in- 
stances of the development of porphyritic constituents may be men- 
tioned garnet, staurolite, andalusite, chloritoid, chlorite, and mica. Such 
porphyritic constituents may occur in rocks which have undergone no 
mass dynamic action, or may be present in rocks which have been al- 
tered by mass dynamic action. In the latter case they are believed, as 
explained by Hobbs* and myself,f to have developed, at least in many 
instances, after mass dynamic action ceased. 
The meaning of the numerous interpenetrations of minerals in the 
recrystallized rocks is rendered clear by the foregoing pages. As a result 
of the disturbance of equilibrium from any cause, a change may take 
place. One mineral may grow. At the same time the adjacent mineral 
may be dissolved. The growth of one in many cases is apparently con- 
ditioned by the solution of the other. Cases of this are the growth of 
* Phases in the metamorphism of the schists of southern Berkshire, by W. H. Hobbs: Bull. Geol. 
Soe. Am., vol. iv, 1893, p. 177. ‘ 
+ Principles of North American pre-Cambrian geology, by C. R. Van Hise: Sixteenth Ann. Rept. 
U.S. Geol. Survey, part I, 1896, pp. 691-694. 
