282 



CONCLUSION. 



east and west for upwards of 350 miles. The 

 whole physical framework of Asia and Africa is 

 projected on lines, which run east and west, or on 

 parallels of latitude. 



A like arrangement prevails over the whole of the 

 great American continent, such is the valley of the 

 Amazon ; these lines are beyond the limits of our 

 present inquiry, and it may suffice to notice that 

 the depression of the North American lake system 

 lies between the same parallels as does that of 

 Asia. 



It may be stated generally that throughout the 

 whole of the northern hemisphere there are certain 

 linear areas of depression which are concentric, or 

 which run parallel to one another, at right angles to 

 the earth's polar axis — they conform to lines of 

 equal curvature of the earth's polar compression. 

 These lines of depression have produced our inland 

 seas and lakes, and determined the river systems 

 connected with them ; the original formation of 

 these depressions often dates back to periods of 

 considerable antiquity in the earth's history. By 

 means of the testaceous remains of the animals 

 which have tenanted the waters of these depressions, 

 the dates of their formation, and subsequent modi- 

 fications of extent and form, can be definitely as- 

 certained. 



These long linear areas have not been produced 

 at once, and though this subject may seem to 

 belong rather to the geological history of the 



