GEOLOGY 
volcanic action of any kind have yet been found here in rocks of this 
age. 
Life of the Silurian Period.—As the Ordovician Period was one of 
great length many important changes in the organic world took place. 
If, to the time required for the evolution of these changes, we add the 
enormously long interval represented by the unconformity ; and, again, to 
these, add the time required for the accumulation of the Silurian Rocks, 
we may be prepared to find that the slow march of organic evolution 
had given rise to many and important developments in the organic world. 
Most of the Trilobites, a group so characteristic of the different zones of 
the Ordovician Rocks, had now died out ; the last of the Graptolites dis- 
appeared near the close of the Silurian Period, and so with various other 
groups not so conspicuous as fossils. On the other hand, the Arthropoda 
developed along a new line, and we find the great water-scorpions, or 
Eurypterids, amongst the dominant forms of invertebrate life. Near the 
close of the Silurian Period true vertebrates of low zoological grade, make 
their appearance. They are represented by several varied types of fishes, 
all primitive creatures belonging to the very lowest ranks of the same 
group as the sharks, skates, and rays of the present time. These Silurian 
fishes can hardly be said to be provided with any true fins except the 
tail. 
Of the plants of the Silurian Period, again, we know but little. 
The few traces of plants that occur in the Cumberland rocks of this age 
were probably of the nature of seaweeds. 
(c) Taking into account the thickness of rock stripped off by de- 
nudation in the interval between the close of the Ordovician Period and 
the commencement of Silurian times, and adding to that the time estimated 
to be required for the formation of the Silurian sediments, the author of 
this article considers that a period of 68,000,000 of years is required. 
DEUTEROZOIC PERIOD 
CarBoniFerous Rocks. 
B. Upper, including the true Coal Measures and the Millstone Grit. 
A. Lower, including the Yoredale Rocks, the Mountain Limestone, and 
the Lower Limestone Shale. 
Op Rep Rocks. 
B. ‘The Upper Old Red Sandstone. 
Great Unconformity. 
A. Traces of the Caledonian Old Red (Granites, etc.). 
Great Unconformity. 
IV. Pre-Devonian Unconrormity.—(a2) Hitherto we have had no 
clue to any of the geographical conditions that prevailed during the 
periods noticed, except those mentioned in the foregoing notes. But as 
we trace the history of Cumberland nearer and nearer to our own times, 
more complete evidence is available, and we are able to get a much 
clearer insight into the nature of several events of the past. This is 
especially true of the period that succeeded Silurian times. The evi- 
dence afforded by rocks of this age in southern Scotland informs us that 
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