352 SCIENCE PROGRESS 



Darwin goes on to point out how fissures parallel to the planes 

 of cleavage would develop during the arching-up of the rock 

 by a mass of molten granite, and " these would be filled with 

 granite, so that wherever the fissures were close to each other, 

 mere parting layers or wedges of the slate would depend into 

 the granite. Should, therefore, the whole body of rock after- 

 wards become worn down and denuded, the lower ends of these 

 dependent masses or wedges of slate would be left quite isolated 

 in the granite ; yet they would retain their proper lines of 

 cleavage, from having been united, whilst the granite was fluid, 

 with a continuous covering of clay slate." 



Here Darwin anticipates the principle of lit par lit injection, 

 put forward by Levy forty-three years later ; but he does not 

 foresee the connexion of this phenomenon with the structure 

 of the adjacent gneiss. In accordance with the views then 

 prevalent, the gneiss of Cape Town was regarded by him as 

 belonging to the shale-series rather than to the granite, and 

 as produced from the former by extreme metamorphism. 



I have had the pleasure of noting on the spot, under the 

 guidance of Prof. Andrew Young, a few further details of this 

 section. The filmy shale-bands are so intimately penetrated by 

 the granite that the large felspar crystals of the latter often lie in 

 these bands as if they were fragments from some older rock. 

 Composite gneiss of many types has been built up along this 

 complex junction. Blocks of shale, completely detached, lie in 

 the granite, and are in various stages of "granitisation " by the 

 development of new minerals, or by the addition of material 

 from the igneous rock. 



Farther north, at Clifton, the granite is eminently gneissic, 

 the dark bands coming up steeply on the face of a cliff cut for 

 the road. Coming from this side towards Seapoint, any one 

 familiar with Donegal or Wicklow, to take only these examples, 

 would be prepared for meeting a mass of shales at no great 

 distance. The gneiss of Clifton is no doubt of composite origin, 

 and represents a large incompletely digested inclusion of the 

 Malmesbury series. 



I have been able to study in greater detail the very similar 

 junction on Carbane, near Glenties, in Co. Donegal, 1 where 

 crystals from the granite appear freely in the flakes of shale. 



1 " On Composite Gneisses in Boylagh, W. Donegal," Proc. R. Irish Acad., 

 vol. xxiv. Sect. B (1902), p. 213. 



