ON CLEAVAGE AND FOLIATION IN ROCKS. 371 



find these laminae crossed by repeated fissures, nearly at right angles. The 

 ' repeated fissures' are, however, often tlie traces of strata, and the nearly 

 vertical lamiu^, so common in these parts of the mountains, are sometimes 

 genuine cleavage*. In the gneissic axis of Mont Blanc the nearly vertical 

 divisions are ' foliation.' Even in our own day the true reading of the 

 structure of the Alps is a difficult problem, and laminas of cleavage are there 

 frequentlv described as layers of stratification. 



The following extracts from ' Travels in the Tarentaise in 1820,1821,1822,' 

 by Robert Bakewell, published in 1823, show that this ingenious author had 

 conceived views nearly approaching those of subsequent writers : — 



" On the eastern side of the valley (Thones), about two miles from the 

 town of Thones, there is a rock which presents an appearance of double 

 stratification, not uncommon in the calcareous mountains of the Alps, and 

 which has frequently induced Saussure to suppose that the vertical strata 

 were placed in junction with other strata nearly horizontal; an error into 

 which he has been led by mistaking very distinct vertical cleavages for stra- 

 tification. On approaching that rock I had little doubt that the strata were 

 vertical, but when I came in front of it I perceived the true strata-seams 

 forming curves, which were intersected at one end by a vertical cleavage. 

 It sometimes happens that the strata-seams are entirely concealed in the 

 perpendicular escarpment of rock by a calcareous incrustation deposited 

 over the face of the rock, and in such instances the cleavages often project 

 and resemble strata so much that it requires great care to avoid error in 

 tracing the true line of dip in the stratification. This probable cause of 

 error is of frequent occurrence in the Alps." — Vol. i. p. 67- 



In the valley of the Arve — " The cleavages on a large scale are often as 

 regular as the strata themselves, and can be scarcely distinguished from 

 them ; and as these cleavages intersect the strata nearly at right angles, this 

 has also led to many erroneous conclusions respecting the stratification of 

 the calcareous mountains of this part of Savoy." — Vol. i. p. 337- 



From the Appendix, vol. ii. p. 423 : — " There are other situations where 

 the calcareous mountains of the Alps present to the hasty observer an ap- 

 pearance of the most irregular and contorted stratification imaginable, which 

 is merely an optical illusion produced by a variety of cleavages in the moun- 

 tain limestone ; some being at right angles to the line of dip, and others to 

 the line of bearing. There is likewise another cleavage in some of these 

 mountains which is curved, and is produced by a tendency to a globular 

 structure in the mass of the mountain. [A mountain in the valley of 

 Lauterbrun, referred to as an illustration.] Near the end of the mountain 

 the true strata-seams are seen, and are nearly horizontal, while farther up 

 the valley several curved perpendicular cleavages present the appearance of 

 thick beds of strata very much bent. In this instance the overlapping of 

 the edges of the strata and the direction of the natural cleavages have nearly 

 concealed the true form of the stratification. Such instances as this are of 

 frequent occurrence in the Alps, and have been the source of many erroneous 

 conclusions, for they have hitherto been but imperfectly understood. This 

 tendency to a globular structure en masse I observed very frequently in the 

 Bernese Oberland. It is altogether independent of stratification, though it 

 has often been mistaken for it; but it has not hitherto been noticed, that I 

 know of, by any geologist that has visited the Alps. The limestone in which 

 I observed the curved cleavage most distinctly is dark coloured, hard and 



* See Renevier, Bull, de la Soc. Vaudoise, 4 July, 1855; Forbes, Travels in t'le Alps; 

 Sharpe, Geol. Proceedings, 1854. 



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