124 Dr. C. Callaway—Pre-Cambrian Geology of Anglesey. 
it is fair to conclude that the adjoining band of “ Arvonian” dips in 
conformity with the Dimetian, and, therefore, below it. 
One-third of a mile E. of Pen y carnisiog, in the “‘ Arvonian ” zone, 
hilleflinta, becoming rather granitoid at joint surfaces, is exposed. 
The “Arvonian” band is to be traced continuously for two miles 
further to the N.E., where, at Glanrafon, E. of Llanbeulan, it is 
represented by a well-marked quartz-felsite. The Dimetian and the 
schist (gneiss) have by this time approached much more closely. 
Fic. 4.—Section two furlongs N.W. of Gwalchmai Church. 
e a pirgad ) i a 
a=Schist. c= Breccia. e= Massive ‘*G.”’ 
b= Felsite. d= Bedded “ G,”’ f=Fault. 
(b). Section N. of Gwalchmat.—One mile further to the N.E., 
four miles N:E. of Ty Croes, is a section which I regard as decisive 
of the true relations of the three groups. It occurs N. of the Holy- 
head Road, two furlongs N. of Gwalchmai. The “ Arvonian” has 
almost disappeared, the Dimetian and the schistose series being 
separated by a hollow only twenty yards wide. ‘To the HE. of the 
hollow, grey gneiss is sharply contorted for two or three feet, and 
then it plunges to the 8.E., in conformity with the uniform dip of 
the schist on this line of strike. To the W. of the depression, the 
Dimetian rises in a low, but steep, escarpment. At the base is a com- 
pact felsitic rock (Note 5, p. 126), with a horny fracture. Fine-grained 
granitoidite also occurs at the same level. Immediately overlying 
the felsite is a band of breccia, three or four feet thick. This bed is 
composed of horny-looking fragments, similar to the basement felsite, 
in a felsitic matrix (Note 6, p.126). It clearly dips to the N.W. at a 
high angle, and passes gradually up into ordinary granitoidite, dis- 
tinctly bedded, with the same dip; which, in its turn, passes up into 
the usual massive granitoid rock (“ G ” of the Survey), in which all 
traces of stratification are lost. Figs. 8 and 4 illustrate the preceding . 
description. 
This section proves that the Dimetian and the schistose group at 
this point are separated by a fault, for they dip steeply in opposite 
directions, the “ Arvonian”’ has almost disappeared, and the schist 
is abruptly contorted at the junction. This dislocation undoubtedly 
runs to the §.W., and holds the same position in Dr. Hicks’s section, 
throwing his “ Pebidian ” schists quite out of stratigraphical relation 
with his Dimetian and “ Arvonian.” That the schists are of Pebidian 
