25 
however a considerable thinning out in thickness when com- 
pared with the Bristol and South Wales Coal-fields. Thus the 
total thickness of the Limestone at Clifton is about 2900 
feet,* in South Wales the formation is from 700 to 1000 feet 
in thickness, while at Purlieu, in the Forest of Dean, it is about 
1102 feet.+ The Millstone grit at Bristol may be taken to 
average 1000 feet thick, that of South Wales 200, whilst at 
Purlieu it is represented by 41 feet of rock. t 
The Carboniferous Limestone rests upon the Old Red Sand- 
stone, and the passage beds between the two formations are 
exceptionally well developed in the road from Ross to Dry- 
brook in what is known as the deep cutting. The Old Red 
Conglomerate is composed of vein quartz pebbles,§ embedded 
in a ferrugino-arenaceous matrix. A section of the passage 
beds, with the greater portion of the Carboniferous Limestone 
which rests upon them, was made by the late Mr. Jounn Jonzs 
and Mr. W. C. Lucy, F.G.S., and published in the Club’s 
Proceedings. || 
The section to which I especially desire to draw your atten- 
tion to-day has been exposed at Morse in the making of a 
railway to Mitcheldean. The Limestone is not shown in 
the cutting, nor is the total thickness of the lowest bed of 
sandstone to be ascertained at this point. As seen in the 
section which accompanies this paper, the bed consists of a 
mottled sandstone, built up of well rounded grains of quartz. 
These average about ‘01 of an inch in diameter; they are 
scratched, and some of them contain cavities. A microscopic 
section of this grit is represented in Fig. I., magnified 22 
diameters. When I first saw this bed a doubt occurred whether 
it was Millstone grit, as the lithological features were different 
from anything which I had seen before, and strikingly so when 
* Proceedings of the Bristol Naturalist’s Society, New Series, Vol. L., 
Part iii., 1875-6, page 316. 
+ Memoirs of the Geological Survey, Vol. I., page 129, 
t Memoirs of the Geological Survey. Vol. I., page 127. 
§ Memoirs of the Geological Survey, Vol. I., page 64. 
|| The President’s Address, 1867. 
