290 Proceedings of the Royal Physical Society. 
the aggregate thickness of the coal-seams in the Upper 
pe achoniievinn Rocks near Manchester, gives us 315,000 years. 
This takes no account of any intermittent action during the 
formation of these coals, nor does it take into account the 
fact that north of Yorkshire all the Upper Carboniferous 
Coals belong chiefly to the Millstone Grit, or at the very 
highest to the Lower Coal-Measures of the Manchester and 
Bristol section. An additional 40 feet of coal would there- 
fore need to be taken into account. 
Recapitulating these figures, we have—time required 
for the formation of 7000 feet of sandstone, at 1 foot 
in 1500 years, . : : . 10,500,000 
7000 feet of argillaceous ade: dle 1 foot in 
3000 years, . ! . 21,000,000 
105 feet of coal, at 1 ee in 3000 F aslusd> ; 315,000 
Total, Upper Carboniferous, 31,815,000 
Possible Unconformity at the Base of the Upper Carboniferous 
ftocks—When I joined the Geological Survey in 1867, the 
present Woodwardian Professor (then Mr T. M‘K. Hughes) in- 
structed me, while mapping the Carboniferous Rocks of north- 
west Yorkshire, to keep a look-out for evidence of uncon- 
formity at the base of the Millstone Grit, 7.e., at the base of 
the Upper Carboniferous Rocks. After a few years field-work 
at these rocks, evidence began to be apparent; and Iam now 
convinced that, in the dales of north-west Yorkshire, and in 
the area extending thence towards Craven, the Ingleboro Grit 
(the bottom bed of the Upper Carboniferous Rocks) does lie 
across, or overstep, the higher beds of the Yoredale Rocks, 
which form the uppermost member of the Lower Carboniferous 
Rocks. Many facts, which seem to me otherwise difficult 
of explanation, are easily accounted for on the supposition 
that this unconformity extends over a wider area still. 
Paleontological evidence is not wanting in confirmation of 
this view. Dr Traquair has repeatedly emphasised the fact 
that the fish fauna of the Upper Carboniferous Rocks has 
hardly any species in common with that occurring in the Lower 
Carboniferous. This becomes a striking and significant fact 
