chap. xin. Cleavage and Foliation, 46 1 



areas with remarkable uniformity, cutting straight 

 through the planes of stratification, 1 but yet being 

 parallel in strike to the main axes of elevation, and 

 generally to the outlines of the coast. The dip, how- 

 ever, is as variable, both in angle and in direction (that 

 is, sometimes being inclined to the one side and some- 

 times to the directly opposite side), as the strike is 

 uniform. In all these respects there is a close agree- 

 ment with the facts given by Professor Sedgwick in his 

 celebrated memoir in the ■ Geological Transactions,' and 

 by Sir R. I. Murchison in his various excellent dis- 

 cussions on this subject. The Falkland Islands, arid 

 more especially Tierra del Fuego, offer striking instances 

 of the lines of cleavage, the principal axes of elevation, 

 and the outlines of the coast, gradually changing 

 together their courses. The direction which prevails 

 throughout Tierra del Fuego and the Falkland Islands, 

 namely, from west with some northing to east with 

 some southing, is also common to the several ridges in 

 northern Patagonia and in the western parts of Banda 

 Oriental : in this latter province, in the Sierra Tapalguen, 

 and in the western Falkland Island, the W. by 1ST., or 

 WNW. and ESE., ridges, are crossed at right angles by 

 others ranging NNE. and SSW. 



The fact of the cleavage-laminse in the clay-slate of 

 Tierra del Fuego, where seen cutting straight through 

 the planes of stratification, and where consequently 

 there could be no doubt about their nature, differing 

 slightly in colour, texture, and hardness, appears to me 

 very interesting. In a thick mass of laminated, feld- 

 spathic and altered clay-slate, interposed between two 



1 In ray paper on the Falkland Islands (vol. iii. p. 267 ■ Geolog- 

 ical Journal'), I have given a curious case on the authority of Gapt. 

 Sulivan, R.N., of much folded beds of clay-slate, in some of which the 

 cleavage is perpendicular to the horizon, and in others it is perpen- 

 dicular to each curvature or fold of the bed ; this appears a new case. 



