SS ee 
eS = ee Pe 
oe el dk 
A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 57 
That a subsidence of this kind actually did take place is shown 
by the fact that each member of the Carboniferous Series creeps 
over the edge of the deposit below it. The Millstone Grit, for 
instance, extends beyond the original boundaries of the Mountain 
Limestone, and the Coal Measures again beyond those of the 
Millstone Grit. This is the phenomenon of over/af~, which has 
been so largely made use of in determining the original boun- 
daries, or in other words the coast-dine of any particular member 
of the Series. 
Up to the present time we have been dealing with sedimenta- 
tion which took place in salt or brackish water; but, after the 
deposition of the masses of sandstones and shales of the Mill- 
stone Grit, the great Inland Sea, now so shallowed by coarse 
sandy shoals and mudbanks, became wholly or partially cut off 
from the ocean, and the water threw down finer sand and clay. 
We now begin to find traces of old land surfaces, which ever 
become more and more frequent. Subsidence was still going on, 
but slowly and intermittently, and the fine clay deposits of the 
alluvial flats were often for long periods together so near the 
surface of the water as to support a thick mass of vegetation, the 
remains of which we now have in our Coal Seams.* 
In order to find anything at all approaching the morasses which 
covered a great part of the surface of the British Isles, and of 
Northern Europe, in the Coal Period, we must look to the 
gloomy cypress swamps of the Mississippi. In the Great Dismal 
Swamp accumulate immense thicknesses of vegetable matter, the 
product of generation after generation of growing trees and 
semi-aquatic plants. These masses of peaty matter owe their 
wonderful freedom from any admixture of sand or silt, to the 
filtering agency of the marginal belt of reeds and brushwood, 
which effectually prevents any sediment from mixing with the 
* These land deposits seem to have taken place in the deltas of large rivers, 
even at a very early period in Carboniferous times, for thin beds of coal are 
found in Northumberland in sandy and detrital deposits, which are actually 
contemporaneous with the Mountain Limestone of the Midlands; it is 
manifest, however, that these could have had but a local extension, and that 
the conditions favourable for the growth of vegetation over extensive areas 
were long subsequent. 
