28G 
THE PHYSICAL GEOGRAPHY OF THE PAST. 
case, as far as I know, of the discovery of a fossil bacterium,* 
and it is a suggestive fact that, whilst in course of untold 
ages its contemporaries high up in the scale of existence have 
undergone enormous change, this lowly organism is to-day 
both morphologically and functionally what it was in the Coal 
Period. 
In order that you may picture to yourselves the relation 
of land and water in the British Islands during the Coal 
Period, I must refer you once more to the map of Lower 
Carboniferous times. You must imagine that all the area 
marked as sea has been converted into very shallow water or 
swampy ground, and that these lagoons have somewhat 
encroached upon the old shore lines, thus reducing the area 
delineated as land on the map. The great Central Island 
still existed, but it was narrowed somewhat, and perhaps also 
split up into a chain of two or three islands. The Southern 
Uplands of Scotland, which stood above the water in Lower 
Carboniferous times, were now submerged, and the island of 
the Lake District became much smaller. Still, on the whole, 
the general distribution of the land was pretty much as it was 
when the Carboniferous Limestone was deposited, and this 
must have been occasioned by the land areas not participating 
to the same extent as the sea bottom in the slow downward 
movement which admitted of the accumulation of so many 
thousand feet of strata. 
This is sufficiently indicated in the case of the Central 
Island, by the great thinning out of all the strata as we 
approach its northern shore. The Coal Measures, for 
instance, in our Ashby Coalfield are about 2,500 feet thick, 
and probably were originally 8,000 feet; these as they 
approached the barrier ridge southward have thinned out in 
North Warwickshire within a distance of only 12 or 14 miles 
to 600 feet. 
But we have independent testimony to the fact that the 
land of our Central Island was comparatively stationary 
whilst the sea bottom was subsiding, and that the amount of 
subsidence increased from the Island towards North Derby¬ 
shire. This evidence, to which but little or no attention has 
hitherto been paid, is as follows:— 
In our Ashby Coalfield the largest and most valuable 
seam is that known as the Main Coal, which consists of two 
beds, the Over and the Nether Coal, with a thickness of 5 and 
7 feet respectively. In the northern part of the Coalfield 
* Becharnp’s observations on his supposed fossil microzyme of the 
chalk —Microzyma cretce —have been found to be erroneous. 
