244 ^^^ POPULAR SCIENCE MONTHLY. 



region about Aslieville. Tlie most inexperienced observer can 

 not fail to note the contrasts. 



To this rule, as to all others, there are exceptions. Some of 

 the highest mountains of the world are composed of Tertiary 

 rocks, and volcanic cones, which vary greatly in age, are charac- 

 terized by similarity of form. 



The dependence of scenic contrast on geologic age may well be 

 exemplified along another well-beaten line of travel. 



If we journey from Paris to Lausanne we find the Tertiary 

 plain of France comparatively monotonous ; but in crossing the 

 Jurassic rocks between Tonnerre and Dijon we find them deeply 

 incised by valleys, adding much to the picturesqueness of their 

 scenery by their high relief. At Dijon we are on the western 

 margin of the valley of the Saone, a basin filled with Tertiary 

 and Quaternary sediments and comparatively uniform in surface. 

 Crossing the Saone Valley we ascend the slope of the Jura Moun- 

 tains, pass their summit, and when we descend their eastern 

 flank, from the southeast there bursts upon the eye a vision of 

 serried mountain peaks, lofty, abrupt in outline, and in most 

 cases capped with snow, looking like curling breakers in a 

 stormy sea. This is the Bernese Oberland. Of this well-known 

 Alpine chain the highest peaks are formed of rocks very old in 

 geologic time. Passing southward beyond the valley of the 

 Phone we may cross the Pennine Alps by the Simplon Pass and 

 descend to the Quaternary plain of northern Italy. Here the 

 contrast is abrupt and easily observed, as is the change from the 

 mountain region of Tyrol to the plain of Bavaria about Munich. 



To multiply these instances is unnecessary ; the writer's object 

 is only to explain these scenic contrasts which have been seen by 

 every intelligent observer. In our own country the Atlantic 

 coast plain and the flat sedimentary plains of the Mississippi 

 basin dift'erentiate themselves from the Appalachian mountain 

 region, and the plutonic masses of the Adirondack chain stand 

 out in bold contrast to the glacial and post-glacial deposits of the 

 western portion of this interesting wilderness. So in the Rocky 

 Mountain region the central masses of Archsean rocks stand out 

 in strong relief above the later formations which border them. 



To elaborate this subject in detail would be to write the geol- 

 ogy of the whole world, a task from which the writer refrains. 



From the fact manifest in the Alps tliat glaciers rarely form till the mean an- 

 nual temperature falls below 27 Fahr., Prof. T. G. Bonney estimates that a fall of 

 20 Fahr. would produce large glaciers in the hill districts of Britain ; one of from 

 this to about 12 Fahr. would bring them back in the various districts on the globe 

 where traces of them have been observed, and in some of these the small size of 

 the vanished glaciers shows that the fall can not have exceeded about 15 Fahr. 



