516 Reviews-—The Geology of Anglesey. 
with a description of the petrology, followed by an account of the 
paleontology, after which the succession is established and a general 
view of the development of the formation throughout the island is 
given, and concludes with such matters as thickness, relations to 
other formations, and an attempt at a restoration of the physiography 
of the period. A separate chapter on the details of local develop- 
ment is provided for those who wish to stady the formation in the 
field, and where necessary another chapter deals with the Tectonics. 
In the treatment of the Mona Complex the order is slightly varied, 
and many sections of the general account are replaced by whole 
chapters. 
For the majority of geologists this account of the Mona Complex 
will be the outstanding feature of the work, and there can be no doubt 
that the decipherment of the succession and structure of this com- 
plicated and often highly crystalline tract is Mr. Greenly’s crowning 
achievement. Work of such nature demands that petrological 
investigation go hand in hand with field work, and its successful 
issue must be ascribed to the author’s capacity to carry out both 
lines of research. Out of a complex in which some seventy rock 
types are distinguished, including representatives of all the commoner 
types of sediments, spilites and associated jaspers, felsites, tuffs, 
and plutonic rocks ranging from serpentines to granites, he is able 
to establish the existence of a number of rock groups. These groups 
occur in several isolated regions, and in varying states of meta- 
morphism, some even developing important changes of facies, but 
by the application of stratigraphical principles based on precise 
mapping, these difficulties have been overcome and a definite order 
of succession made out. Furthermore, the discovery of pebbles 
from one group in the conglomerates of another has given definite 
evidence of the chronological order in which this succession must 
be read. Thus we attain the conception of an original bedded 
succession of six principal subdivisions resting upon an ancient 
floor of yet older foliated rocks of which it is conjectured that certain 
gneisses form the only visible representatives. Into this bedded 
succession has been intruded a varied suite of plutonic intrusions, 
and the whole subjected to powerful regional metamorphism. Three 
special peculiarities of the Mona Complex are emphasized, its high 
sodium and titanium contents, which show themselves in the 
great predominance of albite among the felspars and ilmenite 
among the iron ores, and its persistent green colour due to the 
abundance of anamorphic chlorite. 
That this remote epoch was not without organic life is indicated 
by the discovery of annelid pipes and castings, while the jaspers are 
suspected to be altered radiolarian rocks. 
The succession once established, the author proceeds to 
demonstrate his views on the tectonics of the Complex in a chapter 
of singular interest. The key to the interpretation is found in Holy 
Isle, where the effects of pitch and of two great normal faults combine 
