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A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 51 
Yorkshire, by the east and west plications already referred to, it 
shows some decided indications of altering its character, for we 
now find it containing several beds of shale, or hardened clay. 
Still, however, the limestone predominates, but the deposit has 
not quite the pure character of the limestone further south. A 
little further north still the shales or clays become thicker, and 
the limestones thinner, and, at the same time, the beds of lime- 
stone become divided by beds of sandstone. ‘This progressive 
change continues right into Northumberland, where the massive 
- Mountain Limestone and Yoredale Series of Derbyshire are found 
to be replaced by a mass of sandstones, shales, and thin lime- 
stones, containing in the upper part as many as seventeen seams 
of thin, but workable coal, all deposits of shallow water origin. 
Now this extraordinary but gradually progressive change in the 
character of the beds when traced laterally, can only be satisfac- 
torily explained in one way, when we bear in mind what we may 
call the physics of sedimentation. As we proceed northwards, 
we are leaving behind us the deep sea of Lower Carboniferous 
times, with its clear water and limestone-building creatures, 
and are approaching, through gradually shoaling water, the old 
coast line, and the mouths of the rivers which brought down from the 
old Carboniferous land the sand and mud which now form the 
sedimentary deposits. 
The exact position of this old coast line is indicated by a bed 
which lies at the very base of the Carboniferous rocks of the 
North of England, but which is, of course, not met with in the 
Midlands. It is what is known as a Conglomerate, a rock-like 
mixture of sand and rounded pebbles. It is, in fact, a consoli- 
dated and fossilized sea-beach, and we find it abutting against the 
old shore formed by the Cheviot Hills, from which most of its 
rolled fragments have been derived. 
Although the Cheviots formed land during early Carboni- 
ferous times, we have sufficient evidence to indicate that the 
_ area was an island, and that we have to look still a little 
further North for the coast of that great continent which was 
drained by the rivers of the Carboniferous period. In Lanarkshire 
