658 GLACIER-EROSION THEORY 



of the Sarca. Below Mori it is crossed by a formidable barrier, com- 

 posed of a lofty ridge, thrown off from Monte Balbo. This ridge is 

 traversed by the narrow fissure and gorges of La Chiusa, bounded by 

 mural cliffs like those of the defile of the Avon at Clifton, but greatly 

 more contracted. The river Adige continues its straight course through 

 the defile, to emerge above Volgarne and then pass on to Verona ; but not 

 a trace of an erratic block derived from the head of the valley, or of 

 any other glacier indication, has been detected below the commence- 

 ment of the defile. The ancient glacier of the Adige appears to have 

 refused the gorge, and to have been deflected at right angles to its 

 previous course, to pass westwards into the Valley of the Sarca, and 

 thence into the chasm of the lake-basin of the Garda. The fact is 

 strongly dwelt upon by Mortillet ('Ancient Italian Glaciers,' p. 25), and 

 the rationale of the phenomenon is magnificently illustrated by the 

 chaotic accumulation of the blocks and debris of the secondary moraine, 

 called the Slovino di San Marco, a spot which struck Dante with awe, 

 but which does not yet appear to have attracted the notice of English 

 glacialists, who maintain the erosion-hypothesis of Alpine valleys and 

 lake-basins. 



It has been shown above, that an hypothesis of the Dead Sea having 

 been eroded by glacier-action would be beset, to say the least, by very 

 formidable difficulties. Excluding the facts that it occurs in an area of 

 depression, and on a longitudinal instead of a transverse valley, there is 

 much in common between it and the basins of the Italian lakes, viz. : 

 extreme depth, great length compared with width, and chasm-shaped 

 transverse section. Those who have faith in the order of nature will 

 be strongly disposed to infer that the causes which operated on the pro- 

 duction of the one have had a share in that of the others. 



Each of the Italian lakes, taken apart, offers difficulties to the glacier- 

 erosion hypothesis. The present Valley of the Adige, from its head 

 down to Verona, shows not a trace of a lake-basin. That an enormous 

 glacier formerly descended it, is beyond question ; and equally so that 

 that glacier must have greatly widened and deepened the channel along 

 which it moved. But what is the dynamic rationale of the new function 

 it acquired near the termination of its journey, of ploughing down into 

 the bowels of the earth to a depth of nearly 2,000 feet, and in the case 

 of Lago Maggiore 2,605 feet? Can such a result be mechanically 

 sustained by mathematical calculation applied to the conditions of the 

 problem ? or does it accord Avith the observed effects of existing Alpine 

 glaciers near their terminations ? 



I have already alluded to the difficidty in the case of the Lago di 

 Como. Without pre-existing fissures to guide its course, by what 

 means could a progressive glacier have split, so as to excaA r ate, the 

 diverging branches of Como and Lecco ? Why was the promontory of 

 Bellaggio left intact ? In Lago Maggiore 1 2\ miles intervene between 

 Santa Caterina, where the depth is 2,605 feet, and Santo Calande, 

 where the glacier, emerging from the lake, was delivered on the plains 

 of Lombardy, yielding, as already stated, a gradient of 200 feet per 

 mile. Without reference to the fact that the bottom of the glacier, 

 under the hypothesis in question, must have here performed the mecha- 

 nical duties of an auger or a plough-share, could the vis a tergo, under 

 the conditions of the problem, have propelled the bottom stratum of 



