chap. xin. Cleavage of Clay-Slate. 447 



andesite) forming the great injected axes of the Cordil- 

 lera of Chile. 



The stratification of the clay-slate is generally very 

 obscure, whereas the cleavage is remarkably well defined : 

 to begin with the extreme eastern parts of T. del Fuego ; 

 the cleavage-planes near the St. of Le Maire strike 

 either W. and E. or WSW. and ENE., and are highly 

 inclined ; the form of the land, including Staten Island, 

 indicates that the axes of elevation have run in this 

 same line, though I was unable to distinguish the planes 

 of stratification. Proceeding westward, I accurately 

 examined the cleavage of the clay-slate on the northern, 

 eastern, and western sides (thirty-five miles apart) of 

 Navarin Island, and everywhere found the laminee 

 ranging with extreme regularity, WNW. and ESE., 

 seldom varying more than one point of the compass 

 from this direction. 1 Both on the east and west coasts, 

 I crossed at right-angles the cleavage-planes for a space 

 of about eight miles, and found them dipping at an 

 angle of between 45° and 90°, generally to SSW., 

 sometimes to NNE., and often quite vertically. The 

 SSW. dip was occasionally succeeded abruptly by a 

 NNE. dip, and this by a vertical cleavage, or again by 

 the SSW. dip ; as in a lofty cliff on the eastern end of 

 the island the laminae of slate were seen to be folded 

 into very large steep curves, ranging in the usual WNW. 

 line, I suspect that the varying and opposite dips may 

 possibly be accounted for by the cleavage-laminas, 

 though to the eye appearing straight, being parts 

 of large abrupt curves, with their summits cut off and 

 worn down. 



In several places I was particularly struck with the 



1 The clay-slate in this island was in many places crossed by 

 parallel smooth joints. Out of five cases, the angle of intersection 

 between the strike of these joints and that of the cleavage-laminaa 

 was in two cases 45° and in two others 73 °. 



