Reviews—The Geology of Anglesey. 5 
to bring in higher and higher tectonic horizons. Anomalies are at 
once apparent, which can only be reconciled by postulating very 
extensive inversions, and thus recumbent folding is established as 
the dominant structure of the Complex. Two great master-folds 
with complementary thrusts or slides are recognized from the evidence 
in Holy Isle, while upon other evidence, admittedly less complete, 
a third fold is also postulated. The impulse is throughout from the 
north-west, and amplitudes of the order of 60 miles are suggested. 
Superposed upon these primary structures are secondary and 
tertiary folds and thrusts of extreme complexity, with amplitudes 
which vary from thousands of feet to fractions of an inch, and to the 
stresses causing this later folding the author attributes the wide- 
spread regional anamorphism of the Complex. The apparently 
capricious variation in intensity of the metamorphism is shown to be 
dependent on the position of the beds in the tectonic succession, in 
accordance with the principle that anamorphism is a function of 
depth. To this, however, there is one striking exception, which has 
caused the author much concern before arriving at an explanation 
which he regards as satisfactory. 
On the important question of the age of the Complex he is on 
firmer ground. He proves beyond the possibility of doubt that its 
regional metamorphism was complete, and that denudation had 
cut deeply into its most crystalline members before the basal 
Ordovician conglomerates were laid down. This in itself goes far to 
establish a pre-Cambrian age, but Mr. Greenly has gone further 
and obtained direct evidence from the mainland, where he has 
recognized pebbles from many horizons of the Complex in the basal 
Cambrian conglomerates and Harlech Grits. Thus, both the materials 
and metamorphism of the Mona Complex and—if the author’s 
views upon tectonics be accepted—its recumbent folding are proved 
to be of pre-Cambrian age. The importance of this result to 
workers in other British areas of crystalline schists needs no 
demonstration. 
With some understanding of the fascination which these ancient 
schists have exercised over the minds of so many great geologists, 
we turn almost reluctantly to the later formations. But the author's 
energies show no signs of flagging, and he devotes to their con- 
sideration 421 pages full of interesting facts and_ brillant 
generalizations. 
He commences with the study of a few small outliers of 
unmetamorphosed rocks intermediate in age between the Complex 
‘and the Ordovician. Of these certain volcanic rocks are correlated 
with the volcanic group of Bangor, while some sedimentary patches 
are regarded as the last relics of a former widespread sheet of 
Cambrian deposits, soon afterwards swept away by sub-Ordovician 
denudation. 
Passing to the Ordovician, which after the Complex is the most 
widespread formation in the island, we learn that there seems to be 
