Page:Popular Science Monthly Volume 46.djvu/256

244 region about Asheville. The 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 characterized 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 Mountains, 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 Rhone 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 differentiate 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 Archæan rocks stand out in strong relief above the later formations which border them.

To elaborate this subject in detail would be to write the geology of the whole world, a task from which the writer refrains.

the fact manifest in the Alps that glaciers rarely form till the mean annual 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.