chap. xin. Foliation of Mica-Schist, 455 



tilt, our previously inclined folia can be thrown into 

 any angle between 26°, which is the least possible angle, 

 and 90° ; but if a small inclination be thus given to 

 them, their point of dip will depart far from the north, 

 and therefore not accord with the actual position of the 

 folia of mica-schist on our granitic range. Hence it ap- 

 pears very difficult, without varying considerably the ele- 

 ments of the problem, thus to explain the anomalous strike 

 and dip of the foliated mica-schist, especially in those 

 parts, namely, at the base of the range, where the folia 

 are almost horizontal., Mr. Hopkins, however, adds, 

 that great irregularities and lateral thrusts might be 

 expected in every great line of elevation, and that these 

 would account for considerable deviations from the 

 calculated results : considering that the granitic axis, 

 as shown by the veins, has indisputably been injected 

 after the perfect formation of the mica- slate, and con- 

 sidering the uniformity of the strike of the folia through- 

 out the rest of the archipelago, I cannot but still think 

 that their anomalous position at this one point is 

 someway directly and mechanically related to the in- 

 trusion of this WNW. and ESE. mountain-chain of 

 granite. 



Dikes are frequent in the metamorphic schists of the 

 Chonos Islands, and seem feebly to represent that great 

 band of trappean and ancient volcanic rocks on the 

 south-western coast of T. del Fuego. At S. Andres I 

 observed in the space of half a mile, seven broad, parallel 

 dikes, composed of three varieties of trap, running in a 

 NW. aud SE. line, parallel to the neighbouring moun- 

 tain-ranges of altered clay-slate ; but they must be of 

 long subsequent origin to these mountains; for they 



directed to the west 35° 33' south. Hence the two points of dip on 

 the opposite sides of the range, instead of being as in ordinary cases 

 directly opposed to each other at an angle of 180°, would here be 

 only 86° 50' apart. 



