1901 - 2 .] J. G. Goodchild on Scottish Mineralogy. 
323 
commonly occurs the mineral under notice. One is almost justified 
in regarding these occupants of the druses in granites as represent- 
ing such of the substances which existed in the solutions from which 
the granite originated, which were not, so to speak, needed for the 
building up of the granite itself. If eruptive rocks are really 
formed (as I think they may have been) through the slow solution 
of various pre-existing rock materials, sediments amongst others, 
by the action of thermal waters holding alkalies in solution, the 
formation of these druse minerals can be easily enough accounted 
for. They could, in that case, simply be regarded as the latest 
manifestations of the same hydro-thermal causes as those to which 
the formation of the parent eruptive rock was due. 
It is noteworthy that in all the cases in which the druse is 
simply lined with the crystals, they have developed fully, and 
have retained their own crystal boundaries. But wherever the 
crystals have grown so as to touch those adjacent, allotriomorphism 
has commenced, and the crystal boundaries of both minerals in 
contact disappear. In a few cases it can be shown that a certain 
amount of absorption of the substance of the crystal affected has 
taken place ; and it is noteworthy that nearly all such absorption 
takes place along the crystallographic axis which has previously 
coincided with the direction of greatest elongation during growth. 
This appears to be the case with all the minerals which tend to 
assume a more or less elongated form. Allotriomorphism affects 
elongated crystals of Beryl, for example, almost wholly at their 
ends, and hardly at all round the prism zone, the faces of which 
usually remain idiomorphic. 
In the case of the granite druses which contain Albite, it is not 
at all uncommon to find that the associated potash felspars yield 
on analysis a variable, though usually small, percentage of soda. 
This soda may be regarded as occurring in the condition of Albite, 
intergrown with the potash felspar in some way which has not as 
yet been accurately determined. 
As already mentioned, the commencement of the growth of the 
Albite crystals does not appear to have been begun until after 
the formation of the Quartz and the Orthoclase crystals had been 
completed. This is shown by the fact that the Albite may occur 
upon the crystalline faces of either of those of its associates. In 
