6 J.W. Judd — On Volcanos. 



by the great circular lakes of the Italian peninsula leads us to the 

 inevitable conclusion that they, in common with the numerous 

 similar ones of smaller dimensions, owe their origin to the direct 

 explosive action of volcanic forces, — and that they occupy, in fact, 

 the bottoms of vast craters which have been formed by this agency. 



But, as we have already remarked, it is not by suck direct explo- 

 sive action alone that the volcanic forces are capable of originating 

 those depressions in the surface of the land which constitute lake- 

 basins. Sometimes an ordinary river-valley may have a part of its 

 course dammed up, either by the flowing of a stream of lava across 

 it or by the throwiug up of volcanic cones in its midst. And still 

 more striking are the effects produced on the system of drainage in 

 a district by the subterranean movements which so constantly pre- 

 cede, accompany, and follow the outburst of volcanic forces. By 

 this agency true rock-basins, often of vast dimensions, are formed, — 

 sometimes by local subsidence, at others in consequence of inequali- 

 ties of movement along a line of river-valley. 



It would perhaps be impossible to cite any clearer or more striking 

 illustration of the formation of a rock-basin, capable of containing a 

 lake, by the subterranean action of volcanic forces, than that of Lake 

 Balaton (or the Flatten See, as it is called by the Germans) in 

 Hungary. 



Among all the beautiful lakes which at the present time surround 

 the Alpine system, there is none which equals in size Lake Balaton. 

 It has a length of about 50 miles, and a breadth varying from 3 to 10 

 miles, its area being no less than 420 square miles. Its depth, how- 

 ever, unlike that of many of the Alpine lakes, is not very consider- 

 able, averaging only between 30 and 40 feet. Lake Balaton is, 

 nevertheless, a magnificent sheet of water, and in picturesque beauty 

 is scarcely, if at ail, inferior to any of the more famous lakes of 

 Southern Europe. 



In certain of its features, however, Lake Balaton presents in- 

 teresting points of contrast with the Alpine lakes, and to these it 

 will be instructive to refei'. It does not occupy, like them, a depres- 

 sion in one of the great valleys radiating from the Alps, nor has it, 

 indeed, any visible natural outlet. A number of more or less con- 

 siderable streams flow into it ; but until the Roman emperor Galerius 

 constructed a canal between it and the Sio, a tributary of the Danube, 

 the waters of the lake had no communication with that or any other 

 river. Nevertheless, these waters are almost perfectly fresh, and 

 exhibit only the faintest trace of saline characters. 



It is clear to any one who examines Lake Balaton that it occupies 

 a true rock-basin — that is, an actual depression in the surface of the 

 land — and that its waters are not merely dammed up by superficial 

 accumulations. The rocks which inclose it are, on its northern side, 

 of Triassic and Rhastic age, and, on its southern, of older Neogene 

 (Miocene) date — the latter being about the equivalents in time of 

 the Molasse of Switzerland, which forms the shores of a great part of 

 the Lake of Geneva and of so many other Alpine lakes. On all 

 sides, but at different distances from it, the lake is encircled by hills 



