J, Tf. Judd—On Volcanos. 5 



II. — Contributions to the Study of Volcanos. 

 By John W. Judd, F.G.S. 



On the Origin of Lake Balaton in Hungary. 

 (PLATE I.) 



IN our last chapter^ we referred to the frequency of the occurrence 

 of lakes in districts which contain volcanos that are still active 

 or have only recently become extinct.^ In connexion with this sub- 

 ject, we must also call attention to the interesting circumstance that, 

 wherever the geologist finds evidence of the former action of sub- 

 aerial volcanos, there he almost invariably detects proofs also, that 

 numerous lakes have been formed and successively filled up with 

 sediments. Very strikingly is this fact illustrated among the great 

 series of volcanic rocks, which, during a great portion of the Tertiary 

 period, were being erupted in Central and Southern Europe ; and 

 which form an almost complete girdle surrounding, but lying at a 

 considerable distance from, the great central masses of the Alps. 

 We have in these districts the most unmistakable palseontological 

 evidence that the periods of violent volcanic activity were also 

 characterized by the repeated formation and filling up of lake-basins. 

 We have already shown how a study of the features presented 



1 See Geol. Mag. 1875, Decade II. Vol. II. p. 349. 



2 It has been suggested to me by my friend Mr. Scrope that this fact of the very 

 constant connexion between volcanic faction and the formation of lake-basins would 

 be brought out very clearly and impressively by an estimate of the number of lakes 

 which at ^-esent exist in the Auvergne, together with those which have in very 

 recent times been filled up with alluvium. The large map of that part of the 

 Auvergne included within the Department of the Puy de Dome, prepared by the 

 Abbe Le Coq, lends itself admirably to such a purpose, and I have obtained from it 

 the following results. The area of the Department of the Puy de Dome is only a 

 little greater tha,n that of the English county of Lincoln, yet its surface is studded 

 over with the relics of no less than 276 lakes and lakelets. These may be classified 

 as f oUows : 



1. Crater lakes, either still existing or filled up with sediments, and 



clearly formed by explosive action 18 



2. Lakes formed by the arrest of drainage in a valley by the flowing 



of a lava-stream into it, or by the throwing up of volcanic cones 



in its course .......... 3 



3. Lakes lying among the volcanic rocks and due either to the irregular 



accumulation of volcanic materials, to local subsidences, and other 

 changes of level . . . . . . . . .81 



4. Lakes lying in similar depressions among the rocks of the old granitic 



plateau, generally in the lines of drainage, and owing their for- 

 mation to local changes of level in this formerly violently disturbed 

 district 174 



Total . . 276 



In this estimate I have included only the smaller examples of lakes of very recent 

 date. The exact limits of the larger ones formed in the great river-valleys of the 

 district it is now very difficult to define ; and the patches of lacustrine sediment, 

 filling innumerable depressions both of large and small size, of older date, are greatly 

 obscured by later formed volcanic products or have been to a very great extent 

 removed by denudation. I need only add that, as Le Coq well shows, the district of 

 the Auvergne could never have been the seat of powerful glacial erosion, although 

 the perpetual snow which may have clad the higher parts of the district during the 

 Glacial Period may have contributed to the preservation, though not to the forma- 

 tion, of the lakes in question, in the manner pointed out in this paper. 



