286 Reports and Proceedings— 
up along the cleavages into numerous fragments, which were drawn 
away from each other in the direction of foliation, or might pass 
into chlorite, or chlorite and epidote. The chlorite thus formed 
often passed into biotite, and sometimes the biotite was changed to 
white mica. Where shearing was excessive, chlorite sometimes 
passed directly into white mica. 
Soda-lime felspar was altered to epidote or zoisite, and often to 
calcite. A more important result was the production of muscovite 
in the plagioclase. Much of this felspar was reconstructed in small 
clear crystals or granules. Quartz also was abundantly produced. 
Diorite might thus be converted either into a gneiss with two micas, 
or into a gneissoid quartzite. The granite of the veins passed 
through the usual changes in muscovite-gneiss. 
Other secondary minerals were actinolite (from augite), sphene 
(from ilmenite), and garnet. 
It was contended that the granite-veins were exogenous, because 
they appeared as apophyses from large masses, they had the same 
coarse texture in different varieties of diorite, and they produced 
contact-effects similar to those of intrusive veins, including the 
phenomena of aggregation and enlargement in the minerals of 
the encasing rock. 
Foreign minerals were often introduced by infiltration. Thus, the 
hornblende of a diorite was decomposed into chlorite and iron-oxide, 
which passed for a considerable distance along the shear-planes of an 
adjacent granite, giving rise to a chlorite-gneiss, and the chlorite was 
partially changed to biotite. Hpidote might be introduced in the 
same way. 
Both the diorite and the granite of shear-zones tended by loss of 
bases to become progressively silicified. Most of the liberated bases 
could be accounted for. Analyses showed there was an interchange 
of alkaline bases, soda going to the granite, and potash to the diorite. 
Thus, some of the latter contained almost twice as much potash as soda. 
The evidence collected seemed to prove that the schist-making had 
taken place subsequently to consolidation ; but it was clear, especially 
where the rock was heavily sheared, that the constituents had been 
re-dissolved and reconstructed. Thus, as we followed a diorite into 
the core of a shear-zone, we could see the gradual disappearance of 
shear-planes and other mechanical effects, as well as the progressive 
results of chemical synthesis. 
The secondary origin of the micas and of part of the felspar was 
proved by the fact that they were moulded on decomposition-products, 
such as chlorite and epidote, and upon fragments of hornblende 
crystals, which had been crushed during the shearing, and carried 
away from each other. The mineral changes as here described 
resulted from contact-action plus mechanical force. 
2. “The Northern Slopes of Cader Idris.” By Grenville A. J. 
Cole, Esq., F.G.S., and A. V. Jennings, Esq., F.L.S. 
From the publication of Mr. Aikin’s paper in the Transactions of 
the Geological Survey in 1829 to the second edition of the Survey 
Memoir on North Wales, the relations of the geological and physical 
features of Cader Idris have been pointed out in some detail. ‘The 
