12$ Mr Murchison on tlie Glacml Theory, 



recent observations have shewn, that the blocks constitute the uppermost 

 or last surface deposit in tracts which exhibit, here and there, proofs of 

 having been an ancient bottom of a sea. But without extending his 

 theory to other parts of the world, it does not appear to me, even when 

 confined to the Alps, that M. Necker explains satisfactorily how the gra- 

 nite blocks of Mont Blanc should lie upon the Jura, by any reference to 

 subaerial debacle ; for if we are to imagine the deep hollow of the lake 

 of Geneva filled up with gravel, sand, and mud, and forming an inclined 

 talus from the centre to the flanks of the chain, the subsequent scooping 

 out of this enormous mass of materials involves an intensity of degrada- 

 tion as difficult to believe in as the former extreme climate of Agassiz, by 

 which thousands of feet of snow and ice are supposed to have occupied 

 the same deep valley. I ought not to omit to state, that one of the chief 

 elements introduced by Agassiz into this question, the polished and striated 

 surfaces of the rocks, has not yet been alluded to by this author, but will 

 be treated of in his second volume. 



In the mean time, however he may fail to account satisfactorily for the 

 transport of the very distant great blocks, we have to thank M. Necker 

 for the additional materials, which seem to establish one fundamental 

 fact in reference to the Alpine case, viz., when this detritus was cast off, 

 the gorges and flanks of the chain had nearly the same reference to the 

 central crest as that which now prevails. If this be proved, the theory 

 which depends chiefly upon the supposition, that a great elevation of the 

 centre of the chain broke off" the ice and dislodged the glaciers, is de- 

 prived of its chief basis. In what manner Professor Agassiz can account 

 for the Alps being a great centre of dispersion ivhen at a lower level, is in- 

 deed a part of his theory which is not easily comprehended. On the 

 other hand, whatever we miiy think of M. Necker's hypothesis, it must 

 be admitted that the facts adduced by him support one essential point of 

 the glacialists, by connecting the presence of blocks with the existence 

 of glaciers in the Alps, the former being, as he states, invariably found 

 both in the southern and northern watersheds of those mountains, and at 

 the mouths of the great transverse ravines which lead up to the regions 

 of perpetual snow, and in all such cases he allows that the condition of 

 the blocks is highly indicative of their having once formed part of the 

 " moraines" produced by former glaciers. 



But the important point, that the glacier is the chief source of the ori- 

 gin of erratic blocks, is entirely denied by another antagonist to the theory 

 of Agassiz, who has appeared in the person of M. Godeff'roy.* 



After the observations of two summers in the Alps, this author has be- 

 come convinced that the materials of the so-called moraines have not been 

 derived simply by the glacier from the solid rock in the higher mountains, 

 but are the re-arranged portions only of a great pre-existing diluvial de- 



* Notice sur les Glaciers, les Moraines et les Blocs Erratiques, 1840. 



