506 
THE GEOLOGIST. 
bourhoods of Llanelly, Penllergare, and Lloughor. The middle, or 
Pennant rock series attains its greatest development at Swansea, where 
it is 3,000 feet in thickness, and presents several important beds of 
coal ; but in the eastern portion of this field they are very much thinner, 
and contain little or no workable coal. The summits of the hills which 
bound the parallel valleys on the north crop, are nearly all capped witti 
this grit, which adds much to the peculiar configuration of the country, 
and gives a certain identity of outline to its general features. The 
lower measures are below this rock, and are mostly found on the north 
crop, where they can be most easily and conveniently studied, both 
from the gradual inclination of the beds, and the comparatively small 
amount of disturbance that has taken place amongst them. They are 
also to be found on the south at Pentyrch, in the Taff Vale, Cefn 
Cubwr, near £:idge-end, Olive Moor, near Swansea, and Penclawdd in 
Gower ; but the greater inclination of beds, and the frequent interrup- 
tions to the series, render them less convenient for the geologist. 
Having premised this general outline, I will now proceed to offer a 
few remarks on the fossil remains. As far as I have been able to 
examine the beds of the upper measures, I have not found anything in 
them beyond vegetable remains, which are tolerably plentiful ; but no 
appearance of shells, nor any traces of fish. In the lower measures, 
however, I have been more fortunate, and have obtained both in con- 
siderable numbers. The average aggregate thickness of the beds of 
this series is about forty-seven feet, while that of the respective veins 
is from two to ten feet. In this accumulation of beds — which, with 
underclays, seams of iron-ore, sandstones, argillaceous and arenaceous 
shales, we may roughly compute at fifteen or sixteen thousand feet — we 
have seven or eight zones of animal life, showing periods when either 
brackish water or irruptions of the sea prevailed. As, unfortunately, a 
great difference of nomenclature prevails over the whole basin, and as 
many seams of coal, called by particular names in one district are called 
by a very different name in an adjacent locality, I will limit my descrip- 
tion to one portion of the field, and confine myself to the local names 
of the north-east crop. 
Passing over the marine forms of the mountain-limestone, we first 
come to the millstone-grit, which rather thickens in its course from 
Pontypool to Merthyr. Conformably on this lies a mass of rock of about 
forty yards thick, known as the " Parewell Eock," bo called because 
