472 Basal Strata of the Cordillera, paetil 



In Central Chile, the flanks of the main Cordillera, 

 into which I penetrated by four different valleys, gene- 

 rally consist of distinctly stratified rocks. The strata 

 are inclined at angles varying frcm sometimes even 

 under ten, to twenty degrees, very rarely exceeding 

 forty degrees : in some, however, of the quite small, 

 exterior, spur-like ridges, the inclination was not unfre- 

 quently greater. The dip of the strata in the main 

 outer lines was usually outwards or from the Cordillera, 

 but in Northern Chile frequently inwards, — that is, 

 their basset-edges fronted the Pacific. Dikes occur in 

 extraordinary numbers. In the great, central, loftiest 

 ridges, the strata, as we shall presently see, are almost 

 always highly inclined and often vertical. Before giving 

 a detailed account of my two sections across the Cor- 

 dillera, it will, I think, be convenient to describe the 

 basal strata as seen, often to a thickness of 4,000 or 

 5,000 feet, on the flanks of the outer lines. 



Basal Strata of the Cordillera. — The prevailing 

 rock is a purplish or greenish, porphyritic clay-stone 

 conglomerate. The embedded fragments vary in size 

 from mere particles to blocks as much as six or eight 

 inches (rarely more) in diameter ; in many places, where 

 the fragments were minute, the signs of aqueous depo- 

 sition were unequivocally distinct; where they were 

 large, such evidence could rarely be detected. The 

 basis is generally porphyritic with perfect crystals of 

 feldspar, and resembles that of a true injected clay- 

 stone porphyry : often, however, it has a mechanical 

 or sedimentary aspect, and sometimes (as at Jajuel) is 

 jaspery. The included fragments are either angular, 

 or partially or quite rounded ; l in some parts the 



1 Some of the rounded fragments in the porphyritic conglomerate 

 near the Baths of Cauquenes, were marked with radii and concentric 

 zones of different shades of colour : any one who did not know that 



