684 PROF. T. MSKENNY HUGHES ON THE PRE-CAMBRIAN 
Fig. 4.—Plan of Quarry above Yscuborwen, Caernurvon. 
NOGw Jeo nw eh 
ce’. Lower Caernarvon beds. 
d. Greenstone. F. Faults. 
* Rock much jointed and veined in continuation of direction of dyke. 
a’. Sandstone and grit. 
a'. Conglomerate. \ Cambrian. 
a mile E.S.E. of Llanfairisgaer, where it forms a rugged boss in a 
field N.W. of Bryn. This rock is a coarse crystalline aggregate of 
quartz and felspar, or, as Professor Ramsay has described it, is like 
a granite with no mica. In Anglesey a similar rock passes into a 
granitoid rock with mica. It is doubtful whether any specimen 
taken from the heart of this mass has exhibited clear evidence of a 
fragmental origin. There are many systems of divisional planes, 
and sometimes one set and sometimes another come into prominence 
from their regularity and persistency. 
In the quarries on the N. side of Twt Hill the most bed-like planes 
turn over to the N., or a little W. of N., as if dragged down to the 
fault which brings on the Carboniferous rocks. In the N. corner 
of the Twt-Hill Field quarry (fig. 5) there is a band of decomposing 
grey felspathic rock, fullof more or less regularly developed doubly 
terminated quartz crystals, similar to those found by Mr. Tawney in 
the rocks of St. David’s; and in this a band, from two to eighteen 
inches thick, of hard close-textured siliceous rock, which thickens 
rapidly to the east, with an inclination of about 30° N.N.K. It is 
somewhat interrupted along joints, so as to suggest that it may not 
represent original bedding, but may be due to a kind of indigenous 
vein-structure. On the other hand, this interruption of the 
continuity of the mass at a joint is not at all conclusive against 
its being a bed of sedimentary origin with any one who has ob- 
served the irregular and interrupted manner in which the grits and 
quartzites of Anglesey are thrust in among the crumpled schists 
with which they are associated. 
