A HISTORY OF CUMBERLAND 
Some scraps of evidence obtainable in the Craven area seem to indi- 
cate that the older set of volcanic cones may not have extended far in 
that direction either. But that the later (Upper Ordovician) set was not 
far away from that area is shown by an interesting group of sediments 
which occurs there, and in which are mingled some of the finer products 
of the chief explosions of the volcano, which therefore could not have 
been far distant at this time. 
(e) Life during Ordovician Times.—So far as is known at present 
no animal of higher zoological grade than the Invertebrata had come 
into existence during Ordovician times. The highest forms of life yet 
found belong to the Mollusca, and to that section of the Mollusca (the 
Cephalopoda) which includes the Nautilus of the present day. The 
Arthropoda were represented mainly by some Phyllocarida, and by an 
abundance and great variety of Trilobites, which reached their maximum 
development during this period, and whose different forms have a zonal 
value of much the same character as the Graptolites. Brachiopoda were 
also very abundant. All the other classes of the Invertebrata were 
represented. Of the plants we know as yet but little, and we are not 
likely to know much more, seeing that most of the strata of which re- 
mains exist were formed in the sea or else are of volcanic origin. 
(f) The events briefly summarised in the foregoing paragraphs must 
have required for their accomplishment from first to last a period of time 
of inconceivable length, if we may judge by the importance of the 
changes in both the inorganic and the organic worlds which took place 
in the meantime. Group after group of invertebrate organisms was 
slowly evolved from pre-existent forms, its various species reached their 
maximum of development, gradually passed away, and gave place to others. 
Oceanic areas and land more than once exchanged places. Mountain 
masses were slowly built up, elevated above the sea, and in the course 
of long ages wasted away, and their materials were ¢ dually transferred 
to that cradle of new lands, the ocean floor, there to be again used up in 
the formation of later rocks. Geologists, fully cognisant of all these 
changes, and duly taking into account the rate at which such changes 
are proceeding now, may well be pardoned if they regard the time 
represented by these Upper and Lower Ordovician Rocks of Cumber- 
land as one of enormous length. The author of this article, in his 
address to the Royal Physical Society, has estimated it at 45,000,000 
of years.’ 
III. THe Sequence or Events DuRING SiLur1AN Times.—(a) Over a 
large part of Britain there is evidence that the close of Ordovician times 
was marked by considerable disturbance and slow upheaval of the land. 
In some localities, large areas consisting of the previously-formed rocks 
were upheaved, and exposed for lengthy periods to the wasting influence 
of atmospheric causes, and the process continued until, in some parts, a 
1 See Goodchild, ‘Some Geological Evidence Regarding the Age of the Earth,’ Proc. 
Roy. Phys. Soc. Edin., xiii. p. 302. 
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