chip. xiv. Section by the Poriillo Pass. 483 



to the other end where vertical strata formed snow-clad 

 pinnacles. Even outside the main Cordillera, near the 

 baths of Cauquenes, I observed one such case, where a 

 north and south ridge had its strata in the valley in- 

 clined at 37°, and less than a mile south of it at 67°: 

 another parallel and similarly inclined ridge rose at the 

 distance of about five miles, into a lofty mountain with 

 absolutely vertical strata. Within the Cordillera, the 

 height of the ridges and the inclination of the strata 

 often became doubled and trebled in much shorter dis- 

 tances than five miles : this peculiar form of upheaval 

 probably indicates that the stratified crust was thin, 

 and hence yielded to the underlying intrusive masses 

 unequally, at certain points on the lines of fissure. 



The valleys, by which the Cordillera are drained, 

 follow the anticlinal or rarely synclinal troughs, which 

 deviate most from the usual north and south course ; or 

 still more commonly those lines of faults or of unequal 

 curvature (that is, lines with the strata on both hands 

 dipping in the same direction, but at a somewhat 

 different angle) which deviate most from a northerly 

 course. Occasionally the torrents run for some distance 

 in the north and south valleys, and then recover their 

 eastern or western course by bursting through the ranges 

 at those points where the strata have been least inclined 

 and the height consequently is less. Hence the valleys, 

 along which the roads run, are generally zigzag ; and, 

 in drawing an east and west section, it is necessary to 

 contract greatly that which is actually seen on the road. 



Commencing at the western end of the coloured 

 section [Plate I.] where the R. Maypu debouches on the 

 plain of St. Jago, we immediately enter on the por- 

 phyritic conglomerate formation, and in the midst of 

 it find some hummocks [A] of granite and syenite, 

 which probably (for I neglected to collect specimens) 



