192 H. HICKS ON CAMBRIAN CONGLOMERATES IN 
Cambrian rocks. The evidences on which we based these conclu- 
sions were chiefly such as could be obtained by comparing the 
rocks with those found in other areas which we had examined, 
and by noticing carefully whether any alteration had been produced 
in the neighbouring rocks. We recognized that along one side the 
rocks were entirely unaltered, and that the material out of which the 
latter had been built up may have been, in part at least, derived from 
the denudation of the granitoid rocks; but at that time we found no 
fragments which could be stated definitely to have been derived from 
the rocks in the so-called granite patch. Moreover the actual horizon 
of these conglomerates and grits along the western edge had not 
at that time been made out; and we owe it entirely to Professor 
Hughes that their position as basal Cambrian Conglomerates has been 
clearly defined. In his excellent paper on ‘‘The Geology of Anglesey” 
published in 1881, he has worked out the succession of these rocks 
near Llanerchymedd, by fossil and stratigraphical evidence, and 
has proved conclusively not only that they are the basal Cambrian 
conglomerates and grits, but also that the Tremadoc and Arenig rocks 
follow them in succession. J had an opportunity of examining the 
sections near Llanerchymedd under Prof. Hughes’s guidance, and 
there cannot be the shadow of a doubt that his conclusions are 
correct. If further evidence is necessary, it is furnished in the 
sections we examined to the west of Llanfaelog. In descending order 
are the following rocks. (See section, fig. 1.) Under the blown 
sand to the east of Traeth Crigyll black irony slates of true Arenig 
type are found, with a high dip to the north-west. At 'Ty-croes farm* 
and T'an-y-bryn, black slates and more flaggy beds are seen, still dip- 
ping to the N.W. In the depression which separates these farms 
from Ty-hen, the beds are not exposed, and the appearance here 
would rather indicate a line of fault with the loss of some of the 
strata. The next beds which are met with going eastward are flaggy 
sandstones, and these are underlain at Ty-hen by rather rough con- 
glomerates. Between this point and the Dimetian rocks on the east 
shore of Llyn-faelog, a distance of about half a mile, the rocks are 
mainly flaggy sandstones alternating. here and there with bands of 
conglomerate. The beds all dip towards the north-west, at an angle of 
from 30° to 35°. If there are no repetitions by faults, we have here 
a very considerable thickness of the Lower Cambrian rocks exposed. 
The lowest beds of the conglomerates are within 30 yards of some 
exposures of the granitoid rock, in the field on the east side of the 
lake, and I have no doubt an actual contact may be seen by the 
removal of some of the soil in the depression at the head of the 
lake. The lowest conglomerate at this point isin some respects 
the most interesting and important of the basal conglomerates that 
I have yet examined ; for it shows clearly that when it was deposited 
the granitoid rocks immediately to the east must have been exposed, 
and that they formed the old shore-line on which the conglomerates 
were deposited. Not only is the matrix chiefly made up of an arkose 
material, as is some of the conglomerate at Twt Hill, Caernarvon, 
** ‘Two miles west of Ty-Croes Railway Station. 
