46 BASIC INTRUSIONS 



structure, the felspar crystals lying partly or wholly 

 within the patches of fibrous hornblende derived from 

 augite. The calcite is sometimes sufficiently abundant 

 in the rock to cause it to effervesce like an impure lime- 

 stone when a drop of dilute acid is put on it. The 

 calcite is often seen to partly replace the large crystals 

 of felspar, but most of it occurs in the ground mass of 

 the rock. It is to be looked upon as one product of 

 decomposition of the lime soda felspar which once formed 

 a large part of the rock. Epidote is often a very abun- 

 dant constituent, and is probably derived from the lime 

 soda felspar. Little of the original felspar remains, 

 although the outlines of that which has been altered to 

 other minerals can usually be found in thin sections, 

 and in the case of porphyritic crystals the pseudomorphs 

 are easily seen by the naked eye. 



Dykes of these altered rocks are fairly numerous in 

 all parts of the Cango district ; they are usually only a 

 few feet in width, but are traceable for considerable 

 distances. In the valley of the Nels Kiver in the 

 Eastern Cango there are fifteen dykes in the slates 

 within a distance of two miles, all traversing the rocks 

 parallel with or at a small angle to their strike. In the 

 valley of the river which leaves the Cango through 

 Coetzee's Poort three dykes are seen, the northernmost 

 one is six feet thick, the second over 100 feet, and the 

 southernmost is of much greater size and makes an out- 

 crop nearly a mile in width. This great intrusive mass 

 has been traced for twelve miles along the southern 

 edge of the Cango between Coetzee's and Potgieter's 

 Poorts, forming rather prominent deep red hills (see 



