Mr Murchison on the Gluc'uU Theory. 125 



de Saussiire. In the first volume of a work wliich he is now publishing, 

 M. Necker treats, in great detail, the whole subject of superficial detritus 

 connected with the northern and western watershed of the Alps, and 

 gives us the fruits of many j'ears of observation. Adding very consider- 

 ably to the list of plienomena of transported materials collected by M. A. 

 de Luc, he takes his own illustrious ancestor, De Saussure, as his model, 

 and following in the track of the historian of the Alps, he endeavours to 

 enlarge and improve upon that great observer's suggestions. Pointing 

 out the distinctions between two classes of detritus, viz. one of high an- 

 tiquity and another of modern date, M. Necker contends that the enor- 

 mous masses of the ancient drift or diluvial detritus have a direct con- 

 nection with the actual configuration of the surface, because the chief 

 part of them has been derived from the centre of the chain, the flanking 

 and lower mountains, and even the strata on which it rests, having contri- 

 buted comparatively little to the great advancing body. Examining the high 

 valleys about Chamouni and the foot of Mont Blanc, and finding massive 

 walls from 300 to near 600 feet in height, composed of this ancient dilu- 

 vium in its coarsest form, near the extremities of certain glaciers, he con- 

 cludes that they were once the moraines of glaciers which melted away 

 and retired from them. lie then goes on to suppose that when the re- 

 cession of the glaciers took place (an effect which he refers to the same 

 cause as De Saussure), such transversal moraines formed dykes standing 

 out at some distance from the mountain and barred up lakes formed by 

 tlie melting of the snow and ice. These lakes, at length swollen to ex- 

 cess, are supposed to have burst through the moraine barrier, and to have 

 drifted the materials of which it was composed into the lower countries. 

 M. Necker believes that when these ancient glaciers existed, the Alps 

 were considerably higher than at present, and he judges that such was 

 the case, because the " aiguilles " of Mont Blanc have been lowered very 

 considerably in our own times. Arguing that great blocks are never 

 found at the foot of mountain chains which have not permanent gla- 

 ciers, of what Do Saussure called the '' first class," he cites many nega- 

 tive examples, and brings forward the Pyrenees, where no true erratic 

 blocks are seen, as a proof that the minor or second class glacier^, which 

 there occur, never advanced sufficiently far to dam up water-courses, and 

 thus to form those great lakes, to the letting ofl^" of which and to the de- 

 struction of vast moraines, he attributes the presence of large boulders in 

 the Alps. 



I must, however, remind M. Necker, that if he assumes that all great 

 erratic blocks are to be referred to some neighbouring chain, now the scat 

 of glaciers, he forgets the cases in Scotland and England, and indeed 

 many others, far removed from mountain ranges, and which must be 

 classed, as I shall presently shew, with submarine deposits. Indeed, by 

 far the widest spread of erratic blocks with which wc are acquainted, ex- 

 tending over the plains of Germany and Russia, must have taken place 

 (as I believe at least) when those flat regions were beneath the sea ; for 



