THE FAULT .SVSTI-:.NrS I X SOLTII oF SOl'TII .\I"R1( A. 7,y ^^ 



a dyke of dolerite or a fold of qiiartzite. It is. unfortunately,. 

 impossil)lc in most cases to locate these underground fissures 

 from the surface, so that deep horing in this country is always 

 a matter of great risk. In true artesian areas the pervious strata 

 carrying the water lie more or less horizontally, and the bore 

 must strike them if carried deep enough. Here, where we have 

 to deal with fissures, it is (juite another matter ; from the fact 

 that fissures are for the most part vertical, a bore-hole may be 

 put down within a foot or two of c[uite a large fissure carrying 

 water, luit tlic bore-hole may be dry. 



In the (lee])er ])ortions of tlie earth's crust the tearing 

 action also occurs, but the pressure and jjlasticity of the rocks 

 keep the fissures from opening, and we have potential faults or 

 fissures. The strain along the^e lines is. however, sufticient to 

 allow the passage of mineralising solutions, which find the 

 strained material more easy of penetration than the surround- 

 ing rock. Some extremely interesting examples of this occur 

 in the gneiss zone in the rock-.shaft of the Kimberley Mine,, 

 where the granitic minerals, quartz, felspar, and mica have been 

 introduced along the strain-zones in solution, and have grown as 

 separate crystals in the hornblende schist, which still forms the 

 matrix. In this way shadow}- dykes are formed, made up half 

 of the granitic minerals and half of the original schist. 



The two fault-systems with which we have to deal are those 

 of the post-Karroo and post-Cretaceous movements. The first 

 followed the great folds, which resulted in the coastal mountains 

 of Ca])e Colony, and broke through all rocks up to and including 

 the lower Karroo. The second group is more recent, and is of 

 quite a dift'erent nature to the first, and to this system are 

 due the jiit-faults in which we find the Cretaceous rocks of 

 South Africa. The first system produces a series of slices of the 

 earth's crust, each inclined inwards towards the interior of the 

 land, and ending abruptly on the sea-ward side in a fault; this 

 peculiar structure occurs in its greatest development in the 

 north-west of the Iberian Peninsula, and was called by Suess 

 the imbricate structure, because it resembled in some way the 

 manner in which the scales on a lizard's back follow each other.. 

 It must be understood, however, that the earth's crust scales 

 do not tuck in under each other as in the reptilian scales, but 

 only incline towards each other in the same way; where the two- 

 scales meet there is the break or fault. The second series of 

 faults, because they always run in pairs and let down strips of 

 earth's crust between them, may be called the Trough Fault 

 System. 



The Imbric.\te System of Fault.s. 



It is presumed that the order of sequence of the strata or 

 layers of rock in Cape Colony is known ; at the base there are 

 the Malmesburv beds with intruded granite, overlain unconfor- 



