A HISTORY OF CUMBERLAND 
stones, and with a few relatively thin beds of marine limestone, which 
are wonderfully persistent in character over a very large area. The 
total thickness of this subdivision ranges from 1,500 to 2,000 feet. 
These Yoredale Rocks are of great commercial importance in Cum- 
berland, as being the chief repository of much of the hematite and 
nearly all the lead ores. Their details are therefore given more fully 
on pp. 27, 28. 
The second type of Lower Carboniferous Rocks is found in north 
Cumberland, and is like that which characterizes these rocks through- 
out Northumberland, and indeed most of southern Scotland as well. 
In this type (which, of course, graduates into each of the others) the 
Mountain Limestone consists essentially of a thick pile of sandstones, 
with subordinate shales, and with a few thin and impure beds of lime- 
stone, which represent the landward edge of the thick pile of wedges of 
limestone found on the same horizon to the south-east. Putting this 
statement in another form, we may say that the thick mass of limestone 
found at the head of Edenside gradually gives place, bed by bed, to 
sandstones and shales as the rocks trend from south to north, the change 
affecting the limestones from below upwards, so that the lowest changes 
farthest south and the upper retains its character farthest north. In 
the north Cumberland type the Yoredale Rocks, on the other hand, re- 
tain their general character with very little change. These limestones, 
however, also show a tendency to split up and to pass into sandstones and 
shales, the lowest limestones changing first as they are traced from south 
to north, as in the case of the older group. 
The third type is that occurring in west Cumberland. This de- 
velopment of the Lower Carboniferous Rocks is unlike the others in 
some important respects. The essential difference is due to the fact 
that the chief axis of the delta during Lower Carboniferous times lay 
far to the east of Cumberland. Asa result of these conditions more 
sand and terrigenous materials of other kinds were deposited on the 
east side of Cumberland than over the area where the Lake district is 
now, where the water remained deeper and clearer. Hence the chief 
deposit laid down was thalassic and consisted mainly of beds of lime- 
stone. These, traced from east to west, gradually become thinner, the 
beds thinning away from below upwards, as in the other cases noted, 
but with this difference, that, in the present case, they are not replaced 
by sandstones or shales. The changes just described affect all the Moun- 
tain Limestone and the Yoredale Rocks as well. The result is that 
bed after bed of limestone, from below upwards, thins away as we 
advance from Penrith in the direction of Whitehaven, until nearly the 
whole of the Mountain Limestones have coalesced into one or two thin 
beds. ‘The sandstones and shales of the Yoredale Rocks have likewise 
thinned in the same manner, so that at their westernmost exposure the 
limestones have nearly all come together, and now form one almost 
undivided mass. The so-called Mountain Limestone of west Cumber- 
land is thus of Yoredale age—the underlying beds having thinned away 
26 
