A HISTORY OF CUMBERLAND 
by its uniformity of rate as by the enormously long time required for the 
accumulation of a single inch. There are many analogous phenomena 
recently brought to light in connection with the deep sea oozes of 
modern times. By careful study and comparison of these facts over a 
large area geologists have now obtained sufficient knowledge to be able 
to state with certainty at what particular epoch in the Silurian Period 
any given species of Graptolite lived. Or, conversely, if they find the 
Graptolite, its occurrence informs them unerringly of the geological 
dates of the film of clay in which that particular species was entombed. 
These quiet deep-water conditions remained unaltered through a 
long period of time. In the meantime important changes of the sea 
bottom were in progress elsewhere, and in course of time these gra- 
dually affected the area under consideration. The next change gave rise 
to a deposit which, although evidently formed in quiet and deep water, 
does not appear to have afforded the conditions suitable for the growth 
of organisms of any kind. ‘The deposit in question took the form of 
very fine grey mud, which in some respects appears to correspond to one 
of the grey oceanic oozes of the present day, or, possibly, to the fine 
azoic mud which is slowly accumulating in the depths of the Black Sea. 
In its present compacted and altered condition we know it by the name 
of the Pale Slates—a not altogether appropriate name seeing that, al- 
though characterized by a grey tint, the rocks rarely form what may be 
called slates, in any sense of the word. 
While the deposition of the Pale Slates went on in the tranquil 
depths of the sea over the area now under consideration, coarser sedi- 
ments, laid down in shallower water, were deposited in the areas to the 
north of the Border; and the same occurred also in what is now the western 
part of Wales. The total thickness of the Pale Slates rarely exceeds 600 
feet ; but the deposits found nearer the land attain a thickness in both 
Wales and Scotland of some thousands of feet. It is only near the 
upper and the lower limits that the Pale Slates contain any traces of 
life. 
After this deep-water and azoic episode in the history of Cumber- 
land there followed a long period of conditions of moderate depth, during 
which subsidence went on concurrently with the deposition of mud, clay 
and sand, which, as in other cases, represents the materials worn off the 
land—wherever that may have been—and transported to the sea by the 
agency of rivers. It was during this period the Coniston Flags, Coniston 
Grits, Bannisdale Slates and the Kirkby Moor Flags, etc., were formed. 
The thickness of sediments found in this way cannot be less than 15,000 
feet in the north-west of England, and may have been more even than 
that. 
To-day, these old sediments, indurated and changed in many ways, 
are known by the following names, counting from the lowest upward, 
and have at least the thicknesses stated : Graptolitic Mudstone and Pale 
Slates, 600 feet; Coniston Flags and Grits, 8,000 feet ; Bannisdale 
Slates, 4,000 feet; Kirkby Moor Flags, 3,000 feet. No traces of 
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