20 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 15, 



series of beds which exhibit a slaty structure ; but nowhere can it be 

 better studied than in the Col du Bonhomme ; the force which pro- 

 duced the cleavage acting from below has laminated all the older 

 rocks, but towards the upper limit of its action has only affected those 

 beds whose materials yielded most readily to its influence. 



The rocks of the Col du Bonhomme are coloured in M. Studer's 

 Map of Switzerland as Jurassic deposits of unascertained age ; perhaps 

 attention to the above-described peculiarities of cleavage may enable 

 us to fix their age more nearly. In the passes of the Saanitsch and the 

 Gemmi, and in the neighbourhoods of Meyringen, Grindelwald, and 

 Lauterbrunnen, I observed that all the Lower Jurassic Rocks, and the 

 lower portions of M. Studer's Middle Jurassic Series, were thoroughly 

 intersected by slaty cleavage ; that in the middle portion of the 

 Middle Jurassic Division only the softer beds were slaty, the harder 

 beds alternating with them being free from cleavage, and that in all 

 beds above these there was no trace of cleavage. Now it is more 

 than probable that in each district the cleavage took place at one 

 period, and ceased at the same time throughout the district ; and 

 that the formation in which only certain beds are slaty is in each 

 case the uppermost subject to the cleavage action. Therefore we 

 may infer that the beds of the Col du Bonhomme are of the same 

 age as those of the Saanitsch, &c., which exhibit similar phaenomena 

 of partial cleavage, and which M. Studer has ascertained to belong 

 to the middle part of the Jurassic series. Thus geologists will find 

 that the study of slaty cleavage may sometimes give them new and 

 unexpected assistance in determining the relative age of deposits ; 

 and this assistance will be found where it is much wanted, in rocks 

 of which the study is rendered difficult from the destruction of other 

 evidence by metamorphic action. 



The next point to which I wish to call attention is the conformity 

 between the arrangement of the cleavage planes at the Col du Bon- 

 homme, and that of the folia of the gneiss of Mont Blanc ; the 

 section of the cleavage planes gives the same fan-shaped figure as 

 that of the gneiss, and this on exactly the same line of strike ; for the 

 plane of vertical foliation which cuts the summit of Mont Blanc, 

 produced along its strike, coincides with that of the vertical cleavage 

 of the Bonhomme ; and in the Col de la Seigne, a little east of my 

 section, another plane of vertical cleavage corresponds to the eastern 

 plane of vertical foliation of the Mont Blanc chain. Or, if instead 

 of fixing our attention on the fan-shaped structure, which has too 

 exclusively occupied all Swiss geologists from Saussure downwards, 

 we look to the arches or anticlinal axes, we find that the antichnal of 

 cleavage planes of ISIant Bourant on the west of the Bonhomme cor- 

 responds to that of the valley of Chamounix ; and the anticlinal on 

 the east between the Bonhomme and Chapieux to the central anti- 

 clinal axis of Mont Blanc. This not mere conformity of direction 

 and position of the planes of cleavage and foliation, but actual con- 

 tinuation of the same divisional planes along arches or anticlinals 

 having the same axis, appears to me conclusive proof that the clea- 

 vage of the slate and foliation of the gneiss and schists are portions 



