356 J. G. Goodchild — On Glacial Erosion. 



Vast, then, as are the dimensions of the great erater-lakes of Cen- 

 tral Italy, it is impossible to doubt that they have been formed by the 

 same causes which have originated the numerous others of smaller 

 size, but of similar character, within the same district, — namely, the 

 explosive action of steam disengaged from masses of lava below them. 

 Nor does it, in the case of these vast craters, seem possible to admit 

 of their areas having been enlarged subsequently to their formation by 

 any kind of erosive action. Not only is there no evidence whatever 

 that these craters have been submerged beneath the ocean ; but, on 

 the contrary, the narrow rivers and valleys by means of which the 

 waters of both Bolsena and Bracciano are carried off, as well as the 

 loose cinder cones in the midst of the former, point to an exactly op- 

 posite conclusion. Neither does the action which Mr. Brigham points 

 out as taking place within that vast lake of liquefied rock, Kilauea, 

 namely, the encroachments of the mass of incandescent liquid upon its 

 walls, by which these are slowly eaten back, appear to throw any light 

 upon the formation of the great Italian craters ; so very different in 

 composition and behaviour are the lavas of Italy and Hawaii respect- 

 ively. All theories of an engulphment of the central masses of the 

 volcano completely fail to explain the regular circular form of these 

 depressions, and their striking similarity to those of smaller size, 

 which have evidently been produced by explosive action. 



Nor, when we reflect on the small portion of the earth's surface, 

 and the very short periods concerning which we have any records of 

 the nature and results of the physical changes that have taken place 

 upon it, need we hesitate to admit that paroxysms may have occurred 

 which, though similar in kind, yet exceeded in their degree of intensity 

 any which man may have had an opportunity of witnessing or re-, 

 cording. 



IV. — Glacial Erosion. 



By J. G. Goodchild, F.G.S. ; 



Of H. M. Geological Survey of England and "Wales. 



{Concluded from page 328.) 



Any one who compares the terraces that are shaped by rivers and 

 the sea with the terraces that are found in the Yorkshire Dales must 

 see that in the one case the denuding agent has acted alike upon 

 beds of all degrees of hardness, and has shorn off the edges of the 

 rocks to one level, whether the strata were horizontal or inclined ; 

 in the other case the denuding agent has acted unequally upon the 

 rocks according to their varying powers of resistance, so that the 

 harder beds were left in relief; and, so far from being all shorn off 

 to one level, it would perhaps be difficult to find any one of the Dale 

 District terraces that is not more or less inclined. The bases of the 

 cliffs formed by the one denuding agent are quite level : those left by 

 the other are often inclined many degrees. 



It will perhaps be remarked that the peculiarities of the Dale 

 terraces are just what one ought to expect if they are the result of 



