672 : DR. C. CALLAWAY ON THE - [ Nov. 1902, 
very clearly: the progressive deformation of the hornblende, and 
the alteration of the felspar, being the most prominent features in 
the transformation. 
I came to the Craig-yr-Allor diorite with this information in mind. 
This rock sometimes shows some eaten of its constituents 
anterior to crushing, as Prof. Bonney remarks in his notes to me, 
But this is not the most marked character of the schistose diorite. 
Four slides (373-376) illustrate this point. They were taken from 
the same mass of diorite in Craig-yr-Allor, at distances of a few 
feet one from the other. The first (373) was selected as a compara- 
tively sound diorite, and the others as illustrating the progressive 
change that takes place in it as a shear-zone is approached. 
(373) In parts of the slide the hornblende is aggregated into 
irregular folia, and a little of it is chloritized. 
(374) The rock from which this slide was taken contains some 
veins of granite. The structure is distinctly schistose, flakes of 
chlorite being drawn out into parallel seams. These flakes often 
coincide with cracks (shear-planes), which are further accentuated 
by iron-oxide. A little quartz appears. 
(375) Taken from a point nearer to a plexus of granite-veins. 
The structure is similar to that last-described, but there is more 
chlorite and less hornbiende. 
(376) Within a plexus of granite-veins, large and small, with a 
general parallelism. The diorite contains little hornblende, but 
abundance of chlorite in roughly parallel flakes. Associated with 
the chlorite is some brown mica and a larger proportion of white 
mica. Much of the latter is in minute flakes orientated with 
the chlorite, and apparently formed contemporaneously with it. 
A part of the felspar-crystals contains white mica, but in smaller 
elements, some of it in microliths. Shear-planes in this slide are 
very marked, running parallel to the foliation. They often pass 
through a fiake of chlorite, and biotite sometimes occurs in the 
chlorite just at the shear-plane, a feature that I have often observed 
in the Malvern gneisses. Prof. Bonney agrees that the characters 
shown in this slide are not inconsistent with the theory of the 
action of a haplite on a pre-existing diorite. 
The facts seen in this series of slides are in close agreement with 
the descriptions of my second Malvern paper.’ “The Anglesey 
diorite resembles the medium-black variety (No. 3) at Malvern, 
the granite in both localities is a haplite, and the contact-eftects of 
the haplite upon the diorite are similar. 
The second stage in the process of gneiss-formation is the injection 
of granite-veins and the production of contact-effects. These points, 
anticipated in the description of the last slide (376), must now be 
discussed. 
Nearly everywhere throughout the gneissic ellipse, the diorite and 
its modifications are penetrated by granite-veins. Their frequency 
1 Quart. Journ. Geol. Soc. vol. xlv (1889) pp. 478-87. 
3 
