CHAP. VIII 
DALRADIAN SCHISTS 
127 
There are two groups of rocks iii Anglesey to which a pre-Cambrian 
age may with probability be assigned. In the heart of the island lies a 
core of gneiss which, if petrographical characters may he taken as a guide, 
must certainly he looked upon as Archaean. In visiting that district with 
my colleague Mr. Teall I was much astonished to find there so striking a 
counterpart to portions of the Lewisian gneiss of the north-west of Suther- 
land and Ross. The ■very external features of the ground recall the 
peculiar hummocky surface which so persistently characteiizes the areas of 
this rock throughout the north-west of Scotland. If the geologist could be 
suddenly transported from the rounded rocky knolls of Sutherland, Ross-shire 
or the Hebrides to those in the middle of Anglesey, south of Llanerchy- 
medd, he would hardly be aware of the change, save in the greater verdure 
of the hollows, whicli has resulted from a more advanced state of decom- 
position of the rocks at the surface, as well as from a better climate and 
agriculture. 
When we examine these rocky hummocks in detail we find them to 
consist of coarse gneisses, the foliation of which has a prevalent dip to 
K.N.W. Some portions abound in dark hornblende and garnets, others 
are rich in brown mica, the folia being coarsely crystalline and rudely 
banded, as in the more massive gneisses of Sutherland. Abundant veins 
of coarse pegmatite may here and there be seen, with piiikisli and white 
felspars and milky cpiartz. Occasionally the gneiss is traversed by bands 
of a dark greenish-grey rock, which remind one of the dykes of the north- 
west of Scotland. There are other rocks, some of them probably intrusive 
and of later date, to be seen in the same area ; but they rei^uire more 
detailed study than they have yet received. 
The relation of this core of gneiss and its associated rocks to the second 
group of pre-Cambrian rocks has not hitherto been satisfactorily ascertained. 
The core may conceivably be an eruptive boss in that group, and may have 
acquired its foliation during the movements that produced the foliation of 
the surrounding schists. But it seems more probable that the gneiss is 
luuch older than these schists, though it would undoubtedly participate in 
the effects of the mechanical movements which gave rise to tlieir deforma- 
tion, cleavage and foliation. 
The second grou}) of rocks occupies a large area in the west and in the 
centre and south of Anglesey. The schists of which it consists are 
obviously in the main a clastic series. One of their most conspicuous 
members is quartzite, wdiich, besides occurring sporadically all over the 
island, forms the prominent mass of Holyhead Mountain. There are like- 
wise baggy chloritic schists, green and purple phyllites or slates, and bands 
of grit, wiiile parts of the so-called “ grey gneiss ” consist of pebbly sand- 
stones that have acquired a crystalline structure. That some order of 
seij^uence among these various strata may yet be worked out is not im- 
possible, but the task will be one of no ordinary difficulty, for the plications 
vol. xliv. (1888) 11 . 463. Further references to tlic work of these observers in Anglesey are 
given in Chapter xiii. p. 220 et seq. The Pre-Cambrian areas of Anglesey arc shown in Map II. 
