264 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE _ [VOL. XI 
due to the presence of numerous inclusions arranged in parallel position. 
They are usually of a definite crystalline form, often similar to that of 
the host, but sometimes they are branching or trichitic inclusions of 
varied outline. Now Judd has pointed out” that, while the inclusions 
have a crystalline form, they are isotropic and the form therefore cannot 
be their own. “I am led to the conclusion that the substances forming 
the various enclosures do not consist of any definite chemical compounds 
assuming the regular crystalline form belonging to mineral species but 
that they are a mixture of various oxides in a more or less hydrous con- 
dition, such as hyalite, opal, gothite and limonite, hence their isotro- 
pism, their varieties in colour and their resistance to the action of 
acids’. In other words the inclusions are pseudomorphs. 
It was also shown that while all the inclusions had one parallel pair 
of faces, the rest of their outlines were very frequently irregular, and 
closer examination revealed the fact that in these cases the inclusion 
did not completely fill its cavity. In view of this Judd adds: ‘‘The 
suggestion which seems to me to be most in accord with all the facts 
of the case is that those enclosures are of the nature of negative crystals 
which are more or less completely filled with the foreign substance, 
the enclosures assume the outlines of true crystals, though hey, do not 
of course exhibit their optical properties”’ 
Goldschmidt has shown in the paper dbove referred to that if an 
etched crystal be placed in a saturated solution of the same substance 
the pits themselves attract the dissolved particles and in consequence 
immediately become filled. It follows then that such negative crystals 
cannot have been left in the growth of the host. Judd suggests that 
they may have been due ta the subsequent action of etching, though he 
does not offer any explanation of its occurrence within the substance 
of the crystal and not on its surface. 
Further, schillerization is to be observed only in deep-seated rocks, 
and Judd believes that as the silicates are known to be soluble to a certain 
extent in water at a great heat and under high pressure, it is to this 
action that we are to look for the cause both of the corrosion resulting 
in the production of the negative crystals and of their subsequent filling 
with foreign material. According to the conclusions reached in the 
paper above quoted, schillerization is due to a process of etching which 
attacks the interior of the crystal rather than its surface, and to the 
subsequent filling of the cavities so formed by an isotropic substance 
derived from the mineral itself. 
The only stage in the process left unexplained is the formation of 
29 Quart. Jour. Geol. Soc., 41: 384. 1885. 
