Prof. J. W. Judd—Statical & Dynamical Metamorphism. 247 
than by referring again to the very widely distributed species ortho- 
clase. This mineral, according as it has been formed near the surface 
or at great depths, assumes the special crystallographic habit, the 
lustre, density and other properties which characterize sanidine and 
adularia respectively. But, subsequently to their formation, the 
crystals may have their optical characters completely altered either 
by heat or pressure, so that the position of the optic-axial plane and 
the angle between the axes change to those of anomalous orthoclase 
(orthose deformé). On the other hand, the great mechanical stresses, 
to which rock-forming orthoclases have in some cases been sub- 
jected, has frequently caused them to assume the external angles, 
the internal structure and the optical properties characteristic of 
microcline. 
But there are a number of other changes that orthoclase-crystals 
are subject to, which, though they have certainly been produced only 
in crystals that have existed in deep-seated rock-masses, yet cannot 
be referred to any process of dynamic metamorphism. 
Almost universally, the crystals of orthoclase in such deep-seated 
rock-masses are found to have lost their transparency and vitreous 
lustre, and to have acquired the opacity and pearly lustre, often with 
the red, pink, grey or green tints distinctive of common orthoelase. 
Sometimes the separation by chemical agencies of minute particles 
in the transparent mass has gone on in such a manner as to give 
rise to the scattering of light which results in “opalescence.” We 
have thus produced the opalescent orthoclase, some forms of which 
are known as “ moonstone.” 
If the change results in the formation and infilling by foreign 
deposits of negative crystals lying in their solution planes, then the 
result is an avanturine or schiller orthoclase, to which some of the 
so-called ‘“ sunstones ” must be referred. 
If the structures developed in the crystals by these agencies be of 
ultramicroscopical dimensions, we may have the beautiful interference 
phenomena produced, which are characteristic of iridescent orthoclase. 
The important researches of Stokes and Madan, and especially those 
recently undertaken by Lord Rayleigh, upon the artificially formed 
iridescent crystals of chlorate of potash, promise to throw much 
new light upon the obscure question of the theory of such a play 
of colours as exhibited by certain minerals. 
The chemical changes which take place along certain definite 
planes within a crystal of orthoclase may result, not only in 
developing a particular lustre along these planes, but also in giving 
rise to a tendency to division along them (pseudo-cleavage), and 
we thus get the beautiful variety known as Murchisonite. 
Lastly, the crystal after having undergone any or all of the 
changes above indicated, may, in consequence probably of changes 
of temperature, break up more or less regularly along certain planes 
(‘‘contraction-rifts”’) ; and, in the clefts thus produced, secondary 
deposits of albite or some other form of felspar may be deposited 
giving rise to the varieties known as Perthite and Microperthite. 
Similar series of changes may be traced in the case of many— 
