1854.] SHARPE STRUCTURE OF MONT BLANC. 25 



present vertical position, § 1049, 1050, 1065. The same systematic 

 error runs throughout the whole of Saussure's volumes ; wherever 

 slates occur, their cleavage is almost invariably represented as strati- 

 fication. Had this error died with its author, it would not now be 

 necessary to expose it, but unfortunately it has taken deep root in 

 Switzerland, and is to be found in the most modern geological works, 

 leading to accounts of perpendicular beds in many slightly-disturbed 

 districts. 



Saussure arrived at his conclusion by the following process, which 

 shows the accuracy of his observations, which, even when his con- 

 clusions are erroneous, are always trustworthy and instructive. He 

 starts with this axiom, frequently stated in different terms : " Les 

 pierres feuilletees, de quelque nature qu'elles soient, ont constamment 

 leurs couches paralleles a leurs feuillets." § 1287, also 2326. This, 

 when applied to crystalline rocks, such as gneiss, mica-schist, &c., is 

 perfectly true ; their principal di^dsional planes are invariably parallel 

 to the plates ox feuillets of mica or talc ; so that the direction of the 

 foliation may be seen either in the arrangement of the mica in a hand 

 specimen, or in the apparently parallel divisional planes which intersect 

 whole ranges of mountains. But by the term couches, Saussure 

 implies, as he expressly tells us, stratified beds formed of materials 

 successively deposited from a fluid, § 1882, 2314, including in stra- 

 tified rocks Granits veines, gneiss, and schists primitive and secondary, 

 and therein he confounded the stratification of sedimentary deposits 

 with the foliation produced by crystalline agency. 



The next step follows necessarily from these premises : finding in 

 many slates that the cleavage-planes are lined with plates of mica or 

 talc, Saussure concludes that these planes represent the true bedding 

 of the rock ; and thus he is led, by a remorseless adherence to his 

 principles, to represent most of the slate-rocks visited as standing 

 perpendicularly on end, until, as he himself tells us in § 1050, he was 

 accused of seeing vertical beds in every mountain. 



It is remarkable that Saussure was led into this error from 

 observing the analogy between the foliation of the schists and the 

 cleavage of the slates ; this analogy was afterwards forgotten until 

 Darwin convinced himself that gneiss and mica-schists were not 

 stratified, by a process of reasoning similar to that of Saussure, but 

 built on a correct foundation. Coupling the now well-ascertained 

 fact that the planes of cleavage of slate are not due to stratification 

 with his observation that those planes are analogous to the planes 

 of foliation of gneiss and mica-schist, he drew the true conclusion 

 that the foliation has no reference to stratification* . Saussure was 

 at least consistent in his error, which he arrived at from building 

 correct reasoning upon an unsound basis : but no such compliment 

 can be paid to the English geologists, who, after correctly distinguish- 

 ing cleavage planes from stratification, still continued to class the 

 foliation of crystalline rocks with the latter instead of the former ; 

 thus proposing to unite two phsenomena of totally different origin, 



* Darwin, Geological Observations on South America, chap. vi. 



