10 J. 77. Judd—On Volcanos. 



also lie in a basin formed by volcanic subsidence. It is now ap- 

 parent that this lake is no mere marsli, or collection of lagoons, as 

 soundings near its shores, which are often exceedingly bold and even 

 mountainous in character, have been found to indicate depths up to 

 275 feet. The fact of the existence of this and the neighbouring 

 lakes immediately under the equator will be of course accepted as 

 proving that they, equally with Lough Neagh and Lake Balaton, 

 could not have been formed by the agency of ice during the glacial 

 period. On the other hand, the frequent mention of basaltic rocks 

 as occurring on its islands and shores, and the report of the existence 

 of a number of active volcanos in its proximity, at least suggest 

 that it may owe its origin to the action of the same causes which 

 have formed the lakes in question. But for the final settlement of 

 this question we must await the arrival of fuller details concerning 

 this very interesting lake district of Central Africa. 



We think that the foregoing remarks will be accepted by all as 

 showing that rock-basins — even those of the very largest dimensions 

 — may have had their origin in those changes of level resultiug 

 from the subterranean movements which have accompanied volcanic 

 action, — and that, as a matter of fact, the largest lakes in the British 

 Islands and in the Alpine regions of Europe, respectively, and not 

 improbably the vast sheets of water in Central Africa also, have been 

 so formed, and could not possibly be the result of glacier-erosion. 



It would of course be very easy to multiply to almost any extent 

 the examples of lakes which, like those of Van, Urumiah, etc., in 

 the district south of the Caucasus, are clearly connected with the 

 outburst of volcanic forces ; or of others, like those of Nicaragua, 

 Managua, Maracaybo, and Titicaca, etc., in the Equatorial part of 

 the American continent, which no one can dream of as having 

 been formed by the action of glaciers. 



It may here, however, be necessary to point out that our argument 

 lends no support whatever to the inference that, in those districts 

 where the action of volcanic forces cannot be traced, lake-basins 

 must be the result of other than subterranean agencies. On the 

 contrary, we maintain that movements, precisely similar in character 

 to those which take place in volcanic districts, are constantly occurring 

 on every part of the earth's surface. It is probable, indeed, that the 

 movements which are connected with volcanic activity are often of a 

 more sudden and violent character than those which take place in 

 non-volcanic districts, and that in consequence of this their effects 

 are more strikingly manifest to us. But to suppose that the 

 permanent effects produced in the case of the former are necessarily 

 greater than in that of the latter, would be as unphilosophical as 

 for a geologist to ascribe to sudden floods (the effects of which strike 

 the most casual observer) a greater share in the excavation of valleys 

 than to the unobtrusive but constant actions of atmospheric waste 

 and ordinary transport by streams. 



If we can demonstrate, in the first place, that in those districts 

 where the effects of subterranean movement are most readily traced 

 (namely volcanic areas) rock-basins have certainly been produced by 

 this agency ; and, in the second place, that movements similar in 



