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BOOK III. 
DYNAMICAL GEOLOGY. 
DYNAMICAL GEOLOGY investigates the processes of change at present 
in progress upon the earth, whereby modifications are made on the 
structure and composition of the crust, on the relations between 
the interior and the surface, as shown by volcanoes, earthquakes, and 
other terrestrial disturbances, on the distribution of land and sea, on 
the outlines of the land, on the form and depth of the sea-bottom, 
on marine currents, and on climate. Bringing before us, in short, 
the whole range of geological activities, it leads us to precise notions 
regarding their relations to each other, and the results which they 
achieve. A knowledge of this branch of the subject is thus the 
essential groundwork of a true and fruitful acquaintance with the 
principles of geology, seeing that by the study of the present order 
of nature, it provides a key for the interpretation of the past. 
The whole range of operations in Dynamical Geology may be 
regarded as a vast cycle of change, into the investigation of which 
the student may break at any point, and round which he may travel, 
only to find himself brought back to his starting-point. It is a 
- mmatter of comparatively small moment at what part of the cycle the 
Inquiry is begun. ‘The changes seen in action will always be found 
to have resulted from some that preceded, and to give place to 
others that follow them. 
At an early time in the earth’s history, anterior to any of the 
periods of which a record remains in the visible rocks, the chief 
sources of geological action probably lay within the earth itself. The 
| planet still retained much of its initial heat, and in all likelihood 
was the theatre of great chemical changes. As the outer layers of 
the globe cooled, and the disturbances due to internal heat and 
_ chemical action became less marked, the influence of the sun, which 
must always have operated, and which in early geological times may 
have been more effective than it afterwards became, would then 
stand out more clearly, giving rise to that wide circle of superficial 
_ changes wherein variations of temperature and the circulation of air 
and water over the surface of the earth come into play. 
In the pursuit of his inquiries into the past history and into the 
present régime of the earth, the student must needs keep . mind 
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