1884.] beticeen the Fish and Juja Rivers, South Africa. 3 



sion, beyond the range of map 2). It is mentioned here to note its 

 peculiarities in weathering, being galleried and caverned in all direc- 

 tions into the most fantastic shapes. (A specimen was sent to me from 

 the Izeli as a huge fossil human pelvis, to which it bore a very strong 

 resemblance). This peculiarity of weathering is due to the compara- 

 tively speedy dissolution of the soda contained in the albite, under the 

 influence of damp and moisture. Every change of wind lodges dust 

 in some crevice or irregularity of surface ; this dust retains the mois- 

 ture, which, acting on the soda, corrodes it into a pit which may 

 continue to deepen till it meets a similarly formed pit from another 

 direction ; or a change of wind may lodge dust in another position in 

 the enlarged pit, causing galleries within galleries, and as this goes 

 on at all angles that dust can be collected or moisture retained, the 

 results are more easily imagined than described in words. 



Dyke No. 31 is a large dyke of orthoclase with brush-like diverg- 

 ing tufts of hornblende distributed through it. This dyke forms a 

 very striking feature in the landscape. Looking to eastward from the 

 heights above the Kei on the east side, it presents the appearance of 

 a deep depression on the surface, running in a perfectly straight line 

 through hill and valley without interfering with the general surface 

 drainage of the land, which follows its shortest course to the sea, 

 crossing the dyke at an angle of about 40®. It is marked on our best 

 maps as the " Transkei Gap," with the note ''probably caused by an 

 earthquake." The explanation of this singular appearance is very 

 simple. The felspar of which the dyke is principally composed 

 decomposes more rapidly than the adjoining stratified rocks, and per- 

 mits of greater wear and tear along the line occupied by the dyke. 



A similar depression, occupied by a dyke largely composed of 

 potash-felspar, occurs a little to the south of Cathcart, and may 

 possibly be a continuation of dyke No. 31. 



The crowding of the dykes where my opportunities of observing 

 have been greatest (in the neighbourhood of East London and on a 

 line from the mouth of the Gonubie Eiver to the Kei Bridge) warns 

 me that I have not exhausted their number, and that others may 

 exist that have escaped my observation. Of the 34 numbered in the 

 space of 100 miles, only two of them (Nos. 33 and 34) deviate 

 from the (roughly) east and west direction taken by the other 32 

 dykes. This they do to the extent of 30 deg. They are singularly 

 parallel to each other, and have the same appearance and mineral 

 composition — a coarse-grained dolorite, which yields a large amount 

 of titantic iron in minute crystals on decomposition. They are 

 newer than the other 32 dykes. No. 34 has been traced through 

 Nos. 9, 8, 7, 6, and 5 without suffering alteration. No. 33 cuts 

 through Nos. 26, 25, and 22. This dyke will Le easily recognised 

 as the road passes over in the direction of its length from Draaibosch 

 to the Komgha, and in cutting in the road on the west side leading to 

 the Kei, and about a mile from the bridge, it intersects the centre of 

 the larger dyke No. 26, and presents so marked a difference to the 

 adjoining rocks as to arrest the attention of any one at all observant. 



A rapid en passant notice of these erupted rocks does not permit of 

 minute details. And as all geological correctness is a work of very 

 gradual approximation and very extended research, it would be out 

 of place here to enter into any, based as they would be on a compa- 

 ratively narrow range of observation. But as little or nothing is 



