chap. xiii. Cleavage and Foliation. 469 



axes of elevation, and to the outline of the land : the 

 strike of the axes of elevation (that is, of the lines of 

 fissures with the strata on their edges upturned), ac- 

 cording to the reasoning of Mr. Hopkins, is determined 

 by the form of the area undergoing changes of level, 

 and the consequent direction of the lines of tension and 

 fissure. Now, in that remarkable pile of volcanic rocks 

 at Ascension, which has several times been alluded to 

 (and in some other cases), I have endeavoured to show, 1 

 that the lamination of the several varieties, and their 

 alternations, have been caused by the moving mass, just 

 before its final consolidation, having been subjected (as 

 in a glacier) to planes of different tension, this differ- 

 ence in the tension affecting the crystalline and concre- 

 tionary processes. One of the varieties of rock thus 

 produced at Ascension, at first sight, singularly resembles 

 a fine-grained gneiss ; it consists of quite straight and 

 parallel zones of excessive tenuity, of more and less 

 coloured crystallised feldspar, of distinct crystals of 

 quartz, diopside, and oxide of iron. These considera- 

 tions, notwithstanding the experiments made by Mr. 

 Fox, showing the influence of electrical currents in pro- 

 ducing a structure like that of cleavage, and notwith- 

 standing the apparently inexplicable variation, both in 

 the inclination of the cleavage -laminae and in their dip- 

 ping first to one side and then to the other side of the line 

 of strike, lead me to suspect that the planes of cleavage 

 and foliation are intimately connected with the planes 

 of different tension, to which the area was long sub- 

 jected, after the main fissures or axes of upheavement 

 had been formed, but before the final consolidation of 

 the mass and the total cessation of all molecular move- 

 ment. 



1 See Chapter III. of the present work. 



