518 Reviews—The Geology of Anglesey. 
a complete succession, in which, notwithstanding the nearness of 
Snowdonia, no trace of volcanic activity is found. Save for the 
basal conglomerates and grits, the facies is graptolitic throughout, 
there being a complete passage from an Arenig and Llanvirn of 
typical Welsh character into a Glenkiln and Hartfell of striking 
similarity to the Scottish succession, which extends even to a 
development of the “ Barren Mudstone”’ facies. There is no sign 
of overlap in the formation until the northern coast is reached, 
when all the zones below the Glenkiln suddenly disappear, and we 
are confronted with a distinct northern facies; this phenomenon, 
so long a source of misconception, becomes in the author’s hands the 
key to the interpretation of the post-Silurian earth movements. 
Silurian rocks, once probably widespread, are now preserved only 
in the deep infold of Parys Mountain, which has yielded a rich 
Valentian graptolite fauna, and is also the seat of remarkable 
metasomatism. These phenomena, of which the once famous 
copper ores represent a certain phase, are fully treated in a separate 
chapter. 
Attention is next directed to the great suite of Paleozoic intrusions, 
whose dykes are so conspicuous upon the map. They are shown to 
have a wide range in composition, from the well-known 
“ Hornblende-picrites ”’ to felsites, and cogent reasoning establishes 
their genetic connexion with the post-Silurian earth movements. 
As would be expected from its geographical position, these 
manifestations of the Caledonian crustal disturbances have power- 
fully affected Anglesey, and their effects are discussed in another 
absorbing chapter. For the second time we meet with signs of an 
overwhelming impulse from the north, which becomes increasingly 
evident as we pass away from the mainland. Folding becomes 
isoclina!, cleavage develops, minor thrusts appear, and finally we 
encounter a gigantic rupture curving from coast to coast, along which 
the whole northern region of Anglesey with its distinct northern 
facies, both of Mona Complex and of Ordovician, has been driven 
southwards. for a score of miles. This great dislocation has been 
named the “Carmel Head Thrust Plane”, and the shearing, 
mylonization, and other dynamical phenomena seen along its course 
strongly recall the lesser thrust planes of the North-West Highlands. 
The sudden appearance of the northern Ordovician facies now 
becomes intelligible, and we see that its apparent interstratification 
with the schists of the Complex is due to the rupturing of infolds, 
produced during the same great disturbances. In an interesting 
comparison of these movements with those of the Mona Complex, 
upon which they have been superimposed, the author shows that the 
phenomena are throughout those of a higher tectonic —zone ; 
recumbent folding gives place to fracture, widespread inversion 1s 
absent, and foliation and anamorphism are replaced hy mineral 
changes of a katamorphic nature. 
Of the Upper Paleozoic formations the Lower Old Red Sand- 
