chap. xin. Chiloe and Conception. 457 



though^ where best denned, it ranged within a point of 

 N. by W. and S. by E., dipping either easterly or 

 westerly, at varying and generally very small angles. 

 Hence, from the southern part of Tres Montes to the 

 northern end of Chiloe, a distance of 300 miles, we 

 have closely allied rocks with their folia striking on an 

 average in the same direction, namely, between 1ST. 11° 

 and 22° W. Again, at Valdivia, we meet with the 

 same mica-schist, exhibiting nearly the same minera- 

 logical passages as in the Chonos Archipelago, often, 

 however, becoming more ferruginous, and containing 

 so much feldspar as to pass into gneiss. The folia were 

 generally well defined ; but nowhere else in South 

 America did I see them varying so much in direction : 

 this seemed chiefly caused by their forming parts, as I 

 could sometimes distinctly trace, of large flat curves : 

 nevertheless, both near the settlement and towards the 

 interior, a NW. and SE. strike seemed more frequent 

 than any other direction ; the angle of the dip was 

 generally small. At Concepcion, a highly glossy clay- 

 slate had its cleavage often slightly curvilinear, and 

 inclined, seldom at a high angle, towards various points 

 of the compass ; l but here, as at Valdivia, a NW. and 



1 I observed in some parts that the tops of the laminas of the 

 clay- slate (b of the diagram) under the superficial detritus and soil 

 (a) were bent, sometimes without be- >j on 



ing broken, as represented in the 

 accompanying diagram, which is 

 copied from one given by Sir. H. De 

 la Beche (p. 42, ' Geological Manual') 

 of an exactly similar phenomenon in 

 Devonshire. Mr. R. A. C. Austen, also, 

 in his excellent paper on S.E. Devon 

 (' Geolog. Transact.' vol. vi. p. 437), has described this phenomenon ,• 

 he attributes it to the action of frosts, but at the same time doubts 

 whether the frosts of the present day penetrate to a sufficient depth. 

 As it is known that earthquakes particularly affect the surface of the 

 ground,it occurred to me that this appearance might perhaps be due, 

 at least at Concepcion, to their frequent occurrence ; the superficial 

 layers of detritus being either jerked in one direction, or, where the 



