270 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 25, 



fi cation ended and the cleavage commenced. From the manner in 

 which the clay-slate and sandstone often come up on each side to the 

 base of the quartz ranges, I have no doubt that this rock is a lower 

 and more arenaceous formation metamorphosed. 



The many parallel ranges of quartz in the eastern part of the group 

 extend east and west, but in the more westerly parts they run W.N.W. 

 and E.S.E. : on the west side, however, of the great Sound between 

 the two main islands, there is, according to Captain Sulivan, a fine 

 range, 2000 feet in height, at right angles to the usual direction, and 

 extending N.N.E. and S.S.W. The outline of the indented coast, 

 and the position of the outlying islets, are in accordance with these 

 axes of elevation. The cleavage-planes of the clay-slate strike al- 

 most invariably in the same direction with the quartz ranges : the 

 laminae are either vertical or highly inclined, generally at an angle 

 above 50", and dip either north or south, but most frequently to 

 the south. The coincidence in direction (but not in dip) between 

 the stratification of the quartz and the cleavage of the slate was 

 strikingly seen at the western end of the Wickhara Heights, which 

 bend from their usual east and west line into a W. 35° N. course ; 

 and here at the foot of the hills I found the slate with an almost 

 vertical cleavage striking in the unusual line of W. 30° to 40° N. 

 I may add, that I found on the mainland of South America the 

 cleavage-planes, with a high but variable dip, extending uniformly 

 over extremely large areas, in the same direction as at the Falkland 

 Islands, and in the same line with the prevailing axes of elevation, 

 but intersected at right angles by other subordinate axes : I will not 

 however here enlarge on this subject. 



The beds of sandstone included in the clay-slate in the lower and 

 less troubled parts of the island are either horizontal, or dip in va- 

 rious directions, most commonly to the south, at angles between 10° 

 and 20°. I repeatedly observed that the clay-slate had exactly the 

 same highly inclined cleavage above and below these beds. Where 

 this occurred, the sandstone generally broke, when struck, in the line 

 of the cleavage, and transversely to its own jDlanes of division, and 

 the seams were full of fossil shells: Professor Sedgwick* has remarked 

 the same fact in beds of limestone similarly situated ; and it shows 

 that the molecular arrangement even of these compact rocks has 

 undergone some change. The strike of the cleavage, although co- 

 incident with the main lines of elevation, seems to have no reference 

 to the minor flexures ; and it preserves a remarkable uniformity whe- 

 ther the stratification of the clay-slate (distinguishable only by the 

 intercalated beds of sandstone) has remained horizontal, or has been 

 tilted at small angles in various directions. Captain Sulivan, who 

 was so kind as to observe carefully the cleavage of the rocks, has 

 however given me a drawing and minute description of some clay- 

 slate beds, exposed in a cliff on the southern coast, in which the 

 cleavage in some of the beds strikes perpendicularly without having 

 been in the least influenced by the minor flexures ; whilst in others 



* Geological Transactions, 2nd Ser., vol. Hi. p. 477. 



