Geological Society. 665 
quartz-felsite of the Llyn-Padarn ridge. The greater part, if 
not actually of Bala age, seem to have been intruded before the 
great post-Bala crush-movements, which produced the folding of 
the Lower Cambrian rocks of Llanberis, had entirely ceased. At 
the same time, the evidence does not exclude the possibility that 
some of the intrusions may be of later date. The evidence on 
which these conclusions rest is based mainly on the signs which 
the intrusions exhibit of having been considerably modified by 
earth-pressures, more especially in those portions which protrude 
into Cambrian strata. Petrographical considerations, also, make 
it impossible to separate these rocks from the diabase-sills of 
Bala age occurring farther to the south and south-west of this 
area ; and there is a strong presumption that they represent the 
last residuum of the magma from which the Bala sills were 
derived. 
The north-western portions of the dykes, enclosed in the older 
rocks of the Llyn-Padarn ridge, are comparatively free from 
dynamic metamorphism ; but when traced into the more yielding 
Cambrian grits and slates, they become structurally deformed 
and often so highly sheared as to be hardly recognizable as parts 
of the same dyke. It is suggested that such highly-sheared green- 
stones as occur in the ridge are of still older date, One section is 
described, in which a sheared greenstone is pierced by a tongue 
of felsite about 2 inches wide and 2 feet long. The felsite is 
undistinguishable from that of the rest of the ridge and on the 
borders of the greenstone. Full petrographical descriptions of the 
minerals of the rocks in their altered and unaltered state are 
given : the minerals being taken in the order of their consolidation, 
and the rocks considered in the ‘dynamic’ or crush-zone of the 
sediments and in the ‘ static’ or pressure-zone of the Llyn-Padarn 
ridge. ‘These minerals are apatite; iron-ores altered to sphene 
and leucoxene, and to a mineral which is apparently perowskite ; 
felspars belonging to the albite-anorthite series of one generation 
undergoing ‘ albitization,’ and the formation of felspar-mosaic ; two 
pyroxenes, one possibly rhombic and the other like malacolite, 
eranulitized and associated with secondary albite in the crush- 
zones, or passing into amphiboles and chlorites ; original amphiboles 
rare, but common as actinolite, tremolite, and asbestos alteration- 
products of pyroxene; biotite uncommon; chlorite; quartz; 
epidote; and calcite. In the least-altered rocks the minerals are 
comparatively unchanged; then there is, first of all, molecular 
rearrangement under pressure without movement ; next, myloniti- 
zation and recrystallization ; and lastly, the whole rock becomes 
cataclastic, with partial or complete obliteration of its original 
structure. The gradual appearance of these features towards 
the east is proof that the deforming agency operated from that 
direction. 
