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INTRODUCTION. 
-GeEoLocy is the science which investigates the history of the 
Harth. Its object is to trace the progress of our planet from the 
earliest beginnings of its separate existence, through its various stages 
of growth, down to the present condition of things. It unravels the 
complicated processes by which each continent has been built up, 
and traces the vast geographical revolutions of which each has been 
the site. While seeking to determine the order of the evolution of 
the earth’s great surface-features, it likewise follows, even into detail, 
the varied sculpture of mountain and valley, crag and ravine. 
Nor does this science confine itself merely to changes in the 
inorganic world. Geology shows that the present races of plants 
‘and animals are the descendants of other and very different races 
which once peopled the earth. It teaches that there has been a 
‘progress of the inhabitants, as well as one of the globe on which 
they have dwelt; that each successive period in the earth’s history, 
since the introduction of living things, has been marked by 
characteristic types of the animal and vegetable kingdoms; and 
that, how imperfectly soever they may have been preserved or may 
be deciphered, materials exist for a history of life upon the planet. 
‘The geographical distribution of existing faunas and floras is often 
“made clear and intelligible by geological evidence; and in the same 
way light is thrown upon some of the remoter phases in the history 
of man himself. 
A subject so comprehensive as this must require a wide and 
yaried basis of evidence. One of the characteristics of geology is to 
gather evidence from sources which at first sight seem far removed 
trom its scope, and to seek aid from almost every other leading 
: branch of science. Thus, in dealing with the earliest conditions of 
the planet, the geologist must fully avail himself of the labours of 
the astronomer. Whatever is ascertainable by telescope, spectro- 
‘scope, or chemical analysis, regarding the constitution of other 
heavenly bodies, has a geological bearing. ‘The experiments of 
the physicist, undertaken to determine conditions of matter and of 
energy, may sometimes be taken as the starting-points of geological 
tnyestigation. The work of the chemical laboratory forms the 
foundation of a vast and increasing mass of geological inquiry. To 
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