274 Reviews—The South Wales Coalfield, Milford. 
20. Concxusions or Part III. 
1. Barrell’s investigation of the strength of the earth’s crust, based 
on geological, geodetic, and geophysical data, indicates that the 
depth of maximum flowage by which regional isostasy is maintained 
les within the asthenosphere at a depth far below Haye level 
of eelamaace 
The distribution of temperature in depth (due sic to radio- 
ee energy and partly to the initial thermal state of the earth) 
deduced in Part II is fully in accordance with the mechanical status 
oe to the asthenosphere. 
Mountain-building and associated igneous activity are localized 
by “the radio-active blanket of deep sedimentation. 
4. Rigefaction and tensional faulting and associated igneous 
activity are localized by denudation and relief of pressure. 
5. Becker’s latest method of calculating the age of the earth is 
invalid, and, if it were not, would lead to a figure for the age of the 
earth as great as that suggested by lead-uranium ratios, or to one — 
even greater. 
REV IWS. 
I.—Memoirs oF THE GEOLOGICAL SURVEY. 
Tue Grotocy or tHE Sourn Watxs Coarrrerp. Part XII: Tue 
Country AROUND MILForD, BEING AN ACCOUNT OF THE REGION 
COMPRISED IN SHEET 227 or 1He Map. By T. C. Canrritt, B.Sc., 
F.G.S., E. E. L. Drxon, B.Se., F.G.8., H. H. Tomas, M-A., 
Se.D.; F.G.8., and/O,. 1. Jonus, M\A., D.Sc.) PIGS pp wa, 
185. 1916. Price 2s. 6d. 
N the Magazine for 1915 (p. 171) a notice of the memoir explana- 
| tory to the sheet east of the present one appeared. The general 
character of the geology of the area described in the present memoir 
is similar to that in the preceding one, but there are differences, some 
of which are of special interest. 
The Pre-Cambrian rocks of the Haverfordwest region are continued 
into that of Milford, and are there representative of the same two 
groups, the Joluston plutonic series and the Benton volcanic series. | 
The latter is rich in soda rocks; the former is suspected to be 
intrusive into the latter. A pneissose structure occurs in some of 
the Johnston rocks, and as these are thrust, over Lower Carboniferous 
rocks which exhibit foliation as the result of the thrust, it might be 
suspected that the gneissose structure was also produced by that 
thrust, but the field evidence is against this view, and suggests an 
earlier period for the production of the gneissose structure. Cambrian 
sediments of Lingula Flag age are found as in the area to the east, 
and the general succession of ‘the sedimentary Ordovician rocks in the 
two regions is similar, and therefore requires no further notice. 
An interesting feature i in the area under consideration is the marked 
development of Ordovician volcanic rocks, one certainly and the 
other probably of Arenig age. These are the rocks of the Trefgarn 
and Skomer volcanic series. The Trefgarn Series is developed about 
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