Vice-President’s Address. 291 
when it is realised that the physical conditions under which 
most of the Lower Carboniferous Rocks of Scotland were 
formed were identical with those under which the Upper 
Carboniferous Rocks of the same part of the British Isles 
were accumulated. Mr Robert Kidston is equally emphatic 
upon this point in regard to the plants! We cannot leave 
this unconformity out of account, and yet there are no means 
of forming even an approximate estimate of its duration. 
Lower Carboniferous Rocks—In dealing with these, I shall 
adhere, as before, to the plan of estimating the time by that 
required for the deposition of the maximum thickness of 
marine limestone found during the period—partly because 
we should, I think, take the marine limestones of each 
period on the typical or standard deposit; partly because 
we may safely assume that when we are dealing with marine 
deposits of organico-chemical origin like these, the rate of 
growth must have been the same (or nearly the same) in the 
case of at least all the limestones that have been formed 
from those of the Cambrian Period down to the Foraminiferal 
ooze of to-day. 
The Lower Carboniferous Rocks of the Bristol and other 
areas consist of 2500 feet of marine limestone. This, at the 
rate of 1 foot in 25,000 years, would require 62,500,000 
years. 
Omitting any reference to the Lower Limestone Shales— 
which may be represented in Edinburgh by the greater part 
of the 2000 feet of the red and green beds below the volcanic 
rocks of Arthur’s Seat, we have as a grand total for the 
Carboniferous Period, Upper and Lower, 94,515,000 years, 
It has to be borne in mind that the top of the Coal- 
Measures is probably not left anywhere in Britain. Possibly 
it may be represented on the Continent by some of the beds 
called Permian in Bohemia. 
Upper Old Red.—In places the Lower Carboniferous Rocks 
oraduate downward into the true Upper Old Red—the Lower 
Limestone Shales forming the passage beds from the one to 
the other. This is the case, for instance, near Sedbergh in 
1 See the Vice-Presidential Address before the Royal Physical Society 
in 1894. 
VOL. XIII. x 
