16 



fissures. Occasionally a small channel has been worn through the bottom of these di- 

 viding walls. 



On the SW. , where the structural planes are inclined inwards and the sheets of rock 

 between them are falling off, there are only a few well marked grooves at the upper edge. 

 At one place where the rock has less deeply exfoliated on one side of a cross divisional plane 

 (i. e. one perpendicular to the face) than on the other so as to present a side of a few feet 

 broad at right angles to the face, a channel, about 8 feet deep and 1 foot broad, opening 

 on this side and parallel to the face of the rock , shews clearly that here a portion of 

 the sheets has been loosened, split, and then fallen out. The bottom has afterwards been 

 worn concave from its serving as a rain channel. 



The NW. face , so far as the rock continues nearly perpendicular , presents deep fur- 

 rows , and, when it inclines inwards, these disappear. 



The NE. face, being perpendicular or slighlly inclined outwards presents channels from 

 the summit to the base. 



The NW. and SW. f aces may have originally been grooved to the bottom , as the 

 channels are (on the latter very obviously) decreasing in length by the gradual exfoliation 

 of the rock in planes which intersect them. The lower surface is fresh. Where the chan- 

 nels exist the rock has a black , grey or hoar antique look. If the channels are altogether 

 owing to an operation which is still in progress, the period required to produce them must 

 have been very long, as the weathering now going on must bc extremcly slow. The surface 

 is coverd with such a close vegetable covering , that it must, in great measure, protect it from 

 the mechanical action of the rain. Descending a little to the east of the spot where I had 

 entered the jungle, I examined some large syeni tic masses which rosé from the beach. One 

 of these was divided by a chasm , and on one side, to the breadth of a foot or more, and 

 on the other, to the breadth of 3 or h feet, the rock was a black hornblendic basalt incli- 

 ning to flinty, simi'lar to that before mentioned. This must originally have been a connec- 

 ted zone or dyke about 8 feet broad. The basalt has been freshly quarried and this at one 

 limited place exposed the line of junction of the two rocks. It is sharp and well defined , 

 and on each side the rocks possess precisely the same character which they have at a dis- 

 tance from it. Some of the fragments lying around, howevcr, exhibited the two rocks blen- 

 ding at the line of junction somewhat in the manner of the specimen mentioned abovcp. 12, 

 but frequently thin laminae of the basalt penetrate the crystallized portion of the rock. 

 From the very variable nature of the syeni tes and volcanic rocks of this Island, and the 

 abruptness with which the proportions of the constituents of Ihe same mass often change 

 so as entirely to alter its aspect, I had been previously led to suppose, that the whole 

 belonged to one and the same formation. The appearance of this zone at once pointed 

 to the contemporanity of its origin , and I have no doubt that is was formed in the mode 

 suggested by Mr. Darwin (1), viz. by the opening of a fissure in the syenitic mass while 

 yet viscid into which the most fluid ingrediënt, hornblende, drained from the sides or rosé 



(I) Darwxh on Volcanic Islands , p. 12-4. 



