80 THE POPULAR SCIENCE MONTHLY. 



part of the subject falls rather within the scope of geology, hut I may 

 here refer, in illustration, to the distribution of lakes, the phenomena 

 " of glaciers, the formation of volcanic mountains, and the structure and 

 distribution of coral islands. 



The oriffin and distribution of lakes is one of the most interest- 

 ing problems in physical geography. That they are not scattered at 

 random, a glance at the map is sufficient to show. They abound in 

 mountain districts, are comparatively rare in equatorial regions, in- 

 creasing in number as we go north, so that in Scotland and the north- 

 ern parts of America they are sown broadcast. Perhaps a priori the 

 first explanation of the origin of lakes which would suggest itself, 

 would be that they were formed in hollows resulting from a disturb- 

 ance of the strata, which had thrown them into a basin-shaped form. 

 Lake-basins, however, of this character are, as a matter of fact, very 

 rare ; as a general rule, lakes have not the form of basin-shaped syn- 

 clinal hollows, but, on the contrary, the strike of the strata often runs 

 right across them. My eminent predecessor, Professor Ramsay, divides 

 lakes into three classes : 1. Those which are due to irregular accumu- 

 lations of drift, and which are generally quite shallow ; 2. Those 

 which are formed by moraines ; and, 8, those which occupy true basins 

 scooped by glacier-ice out of the solid rock. To the latter class 

 belong most of the great Swiss and Italian lakes. Professor Ramsay 

 attributes their excavation to glaciers, because it is of course obvious 

 that rivers can not make basin-shaped hollows surrounded by rock on 

 all sides. Now, the Lake of Geneva, 1,230 feet above the sea, is 984 

 feet deep, the Lake of Brienz is 1,850 feet above the sea, and 2,000 

 feet deep, so that its bottom is really below the sea-level. The Italian 

 lakes are even more remarkable. The Lake of Como, 700 feet above 

 the sea, is 1,929 feet deep. Lago Maggiore, 685 feet above the sea, is 

 no less than 2,625 feet deep. It will be observed that these lakes, like 

 many others in mountain regions, those of Scandinavia, for instance, 

 lie in the direct channels of the great old glaciers. If the mind is at 

 first staggered at the magnitude of the scale, we must remember that 

 the ice, which scooped out the valley in which the Lake of Geneva now 

 reposes, was once at least 2,700 feet thick ; while the moraines were 

 also of gigantic magnitude, that of Ivrea, for instance, being no less 

 than 1,500 feet in height. Professor Ramsay's theory seems, there- 

 fore, to account beautifully for a large number of interesting facts. 



Passing from lakes to mountains, two rival theories with reference 

 to the structure and origin of volcanoes long struggled for supremacy. 

 The more general view was that the sheets of lava and scorire which 

 form volcanic cones such, for instance, as Etna or Vesuvius were 

 originally nearly horizontal, and that subsequently a force operating 

 from below, and exerting a pressure both upward and outward from a 

 central axis toward all points of the compass, uplifted the whole strati- 

 fied mass, and made it assume a conical form, giving rise at the same 



