The South Wales Coalfield. 515 
The area of Coal-measures included in the maps before us and 
described in these Memoirs was originally surveyed by Sir William E. 
Logan ; and the results of his labours were generously placed at the 
service of De la Beche, who surveyed portions of the Lower 
Carboniferous rocks and Old Red Sandstone. Aid was also received 
from John Phillips among the Lower Paleozoic rocks. 
The results of the recent survey on the six-inch scale show a great 
advance in methods and details, in the determination of the numerous 
lines of fault, and in the mapping of the superficial deposits. These 
advances are shown perhaps more conspicuously in the Ammanford 
area, where Cambrian (Tremadoc) rocks have been recognized, while 
no less than eight subdivisions, together with limestone bands, are 
represented in the Ordovician rocks, five subdivisions are shown in the 
Silurian, six in the Old Red Sandstone, and many in the Carboniferous 
rocks. In this great series of formations it is estimated that the 
thickness of strata amounts to nearly five miles. We have been 
a little puzzled to find Ammanford on the map, but have succeeded in 
doing so with the aid of a gazetteer wherein Ammanford is mentioned 
as a hamlet with ‘‘ coal-mines, also paint and oilworks.” As a matter 
of fact, there is no town of note in the area, and the population is 
somewhat scattered along the principal valleys, such as the Tawe 
between Ystradgynlais and Pontardawe. . 
Mr. Cantrill and Mr. Thomas have largely added to our knowledge 
of the stratigraphy and paleontology of the Lower Paleozoic rocks. 
The Llandeilo Series is described with the spelling Llandilo ‘‘for the 
sake of conformity with the New Series Ordnance Maps,” a change 
which hardly makes for uniformity, as it is inconsistent with the 
generally accepted spelling of geologists. 
A table and local lists of Lower Paleozoic fossils are given, and 
a method has been devised whereby the precise locality, the collector, 
and the destination of every specimen have been recorded. It must 
be admitted that these important particulars are not of much value in 
the memoir, except when it is used in the Museum at Jermyn Street, 
where are preserved the specimens and also the six-inch maps on 
which the localities are indicated. 
Interesting accounts are given of the changes in the lithology of the 
Old Red Sandstone and of the overstep of its basal portions across the 
Silurian outcrop. The dominant features in the region are great 
plateaus formed by the Millstone Grit, which rises in Black Mountain 
to a height of 2,076 feet, and by the Pennant Grit, which locally 
reaches a height of 1,371 feet. Reference is made to the ‘plastic 
clay’ in the Millstone Grit, a fine mealy sand due to the decomposition 
of chert, which was formed from very fine siliceous sediment, partly 
of clastic quartz grains and partly of organic material. Full 
particulars relating to the Coal-measures are given by Dr. Strahan. 
The lower seams of coal, traced along the ‘ North Crop,’ from the 
Tawe valley through Ammanford and the Llandybie district to the 
Gwendraeth valley, are all anthracitic ; whereas in the ‘South Crop’ 
the lower seams ‘‘-are all bituminous, and there is little doubt that in 
the intervening ground they pass through the intermediate stage of 
steam-coal.”’ 
