GEOLOGY 
thickness of many thousands of feet of the older rocks was stripped off. 
Thus rocks of very different ages came to be exposed at different parts of 
the surface. At the conclusion of this period, which must, if we may 
judge by what took place in the meantime, have been a period of 
immense length, there began a second great period of subsidence be- 
neath the ocean, and the deposition of a new set of strata. The rocks 
referred to are those which now form most of the southern part of the 
Lake district. They are exposed here and there, also in some parts of 
Cumberland, and therefore call for notice here. 
(4) The earliest chapter in the history is recorded in an old bed of 
shingle, which evidently marks the rolling and wearing action of the sea 
upon the loose fragments of rock which were present on the surface as 
the land quietly sank beneath the waves. 
Then follows a stratum of great interest, thin though it be—the 
well-known Graptolitic Mudstone. This is a bed of what was originally 
fine mud, evidently formed at a great depth below the level of the sea, 
and in very quiet water, far beyond the influence of tides or currents, 
and outside the zone of deposition of any muddy outflows brought by 
rivers from the land. On the bed of fine clay which slowly accumu- 
lated on the sea floor, there lived one set after another of those curious 
organisms already referred to as Graptolites. No doubt these, like their 
predecessors, lived in little colonies, each moored to the bottom in 
much the same way as seaweeds are attached to stones and shells on the 
sea floor of the present day. But beyond the fact that they pertained 
to the same subdivision of the animal kingdom as those which preceded 
them, these and their predecessors had but little in common. Every 
one of the older forms that had come into existence had gradually died 
out, and those which lived during the earlier part of the Silurian Period 
were different in many essential particulars from the graptolites of Ordo- 
vician times. A few Brachiopods, and some hardy Crustaceans, lived in the 
ocean depths along with the Graptolites. There are many good reasons 
for believing that one generation of these Graptolites succeeded another 
for a very long time without undergoing any marked change in character, 
or without becoming extinct. Moreover, in the quiet depths of the 
ocean, where the conditions remained uniform, as they usually do now 
over a very large area, the same species of Graptolites were to be found 
far and wide wherever the nature of their surroundings remained uni- 
form. For this reason as well as others Graptolites have been found to 
be of great value as affording a clue to the particular chapter of the 
geological record which they represent. To put this statement into 
another form: these Graptolites lived over very extensive areas of the 
sea bottom, but they throve best only where the water was perennially 
clear, or, in other words, where only a very thin film of sediment found 
its way to the ocean floor in the course of a century. In many cases it 
would appear that the chief deposit there consisted of the remains of 
the Graptolites themselves, mingled with a very small proportion of 
extremely fine mud, the deposition of which was characterized as much 
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