Prof. J. W. Judd—Statical & Dynamical Metamorphism. 245 
between the effects of Eee and dynamical metamorphism 
respectively. 
Setting aside the more purely mechanical effects which may 
precede or accompany dynamical metamorphism—such as cleavage, 
jointing, the crushing or deformation of included fragments, the 
stretching of rocks, and the production of “mylonitic” bands—the 
results of this agency may be classified as follows :— 
First.—The inducement of metamorphoses in the constituent 
minerals of a rock. That minerals are capable, without losing 
their identity, of undergoing remarkable metamorphoses, whereby 
their chemical, crystallographical, optical and other physical 
properties may be modified within certain—though often indeed 
very wide—limits, is now beginning to be recognized by both 
mineralogists and geologists. The production of twin-lamellation 
in many minerals by internal and external stresses, and the defor- 
mation of crystals by these means, so that they assume not only the 
external angles, but the internal structure and the optical properties 
of complex twins belonging to a system of lower symmetry than their 
own, are now well-recognized facts. An admirable illustration of this 
action is afforded by the production of microcline from orthoclase. 
The beautiful experiments of Des Cloizeaux, Dufet and Bucking on 
this same mineral species, orthoclase, have shown that both by heating 
and by pressure the position of the optic-axial plane, and the angle 
between the axes may undergo temporary changes; while, if the 
temperature or pressure pass beyond a certain limit, these changes 
become permanent. The study of the minerals of the enstatite-group 
has shown that as certain chemical changes go on within their 
crystals, these, while retaining their orthorhombic symmetry, may 
undergo the most remarkable modifications, not only in their colour, 
pleochroism, and absorption, but also in their index of refraction, in 
the sign and intensity of their double refraction, in the position of 
their optic-axial plane and in the angle between their axes; and 
further, that, changes in specific gravity, in hardness and in fusibility 
may accompany these several modifications. 
That the form and position of the ellipsoid of elasticity in a crystal 
may be altered temporarily and even permanently by the action of 
heat, pressure, or chemical agencies, is a phenomenon for which we 
ought to be fully prepared; and it is upon the form and position of 
this ellipsoid that the so-called optical “constants” of a mineral 
depend. 
That the metamorphoses which have been more or less fully 
investigated in the case of a few species, like orthoclase, enstatite, 
pelenite, etc., are not confined to those species must be alent to all 
who have been in the habit of studying the so-called “ optical 
anomalies” of minerals; and such “anomalies” are especially fre- 
quent in the case of the common rock-forming species. When we 
remember the nature of the forces to which the constituent minerals 
of rocks must have necessarily been subjected in many cases, we 
shall cease to wonder at this result. 
Secondly. As there must be, in every case, a limit beyond which a 
