124 GE( LOGV AXD MIXERALOGV OF NATAL. 



to it in the case ot the gneiss. Xo hne of demarcation is visible 

 between the underlying granite and the gneiss. The one series 

 passes u}) imperceptibly into the other. The foliated structure 

 seems to take its origin in the granite, .and passes up through several 

 degrees of regularity, until it appears in the most typical form in 

 the gneiss. As a red coloured gneiss is invariably found overlying a 

 red-coloured granite, and a white gneiss a white granite, coupled 

 with the above facts on the foliation, I think it reasonable to con- 

 clude that the gneiss is a portion of the original granitic magma 

 which has suffered under the effects of the metamorphic influences. 

 Fine bands of rock of a quartzose nature are to be found impregnat- 

 ing the gneiss in a direction })arallel to the line of foliation. 



Resting on these gneisses are rocks of great thickness but of a 

 softer and somewhat more friable nature. There is likewise no true 

 line of demarcation between them and the underlying gneiss. They 

 appear to have suffered to a greater extent from the effects of 

 metamor]:)hic influences than the gneiss. They have the same 

 mineralogical comjwsition as the underlying series, but their 

 felspathic constituent has suffered more from decomposition. The 

 occurrence in them of mica is most marked. The mica is the 



variety known as sericite. This has been derived from their felspar 

 constituent. These rocks are very much jointed. The joints,, 

 which are of various degrees in fineness, are to be found mostly 

 running in the direction of dip. but there are other systems of joints 

 traversing the series in an irregular manner such as to lead one to 

 believe that they must have been arranged round centres of disturb- 

 ances. Along the joint planes are often to be found scales or films of 

 silver}' sericite covering particles of quartz or felsi^ar ; and in other 

 joint planes which appear to widen out as they reach the top of an 

 outcrop, the whole crevice has been filled up by very fine white clay 

 or kaolin. This has been filled in b}^ percolating waters. At the 

 present time enormous quantities of water are to l:>e found percolating" 

 through these rocks by means of these joint })lanes. Thesa 

 micaceous schists have a greater degree of fissility along the planes of 

 foliation than the gneiss. There is undoubted evidence that these 

 Archaean rocks have been intruded l^y basic intrusions of different 

 ages. Near Kentterton, Natal, in a railway cutting, is to be 

 found an instance of a typical doleritic rock intruding these ancient 

 rocks. At a few other places I have seen the same thing but 

 unfortunately the basic intrusive rock has been in a most advanced 

 state of decomposition. Near Dumisa, Natal, is to be found a d\ ke- 

 like prolongation in the granite and gneiss. This basic rock, of 

 much older date than the doleritic type, had been intruded into 

 the granite but it has undergone complete alteration and appears- 

 now as a chlorite schist. It must have suffered from metamorphic 

 influences during the same period that the granite and gneiss and 

 schist did. I now refer to veins of pegmatite which occur 

 abundantly in these ancient rocks. They appear to belong to two 

 separate i)eriods. Those that belong to the later period appear to 

 have suffered no changes other than ordinary decomposition, and 

 hence 1 conclude that they were formed at a later period in the earth's 

 history than that at which the gneiss came into existence. They 



