236 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 4, 



give way to pyrites, which are contained either in fissures between 

 masses of shghtly decomposed felspathic granite (see specimen No. 

 78), or diffused in grains in the substance of the granite (specimens 

 83 to 85). 



In fact, the form.ation of all these ores seems to be the result of a 

 process of infiltration, by which the original constituents of the rock 

 are gradually removed, and their place occupied by silicates, oxides, 

 or purple and yellow sulphurets, as the case may be ; the silicates 

 and oxides generally occurring on the surface, the sulphurets below*. 

 Masses of oxide of iron in a state to be acted on by the magnet are 

 often found on the surface of these metallic axes. Sulphuret of 

 molybdenum is found accumulated with copper-pyrites at Concordia 

 and at Kildunern ; and tungstate of lime, in a lump of about a pound 

 weight, was found enclosed in a mass of red oxide of copper, in 

 Springbok Mine. Manganese, too, has been found in various parts 

 of the country. And near Gams I found green oxide of chrome 

 accumulated in small quantities between the layers of gneiss in an 

 *'axis of disturbance''^ . 



In some few cases the *'axes of disturbance" are traceable for but 

 a very short distance. At Nababeeb, for instance, one of the axes is 

 seen on one wall of a ravine, but I could find no trace of it on the 

 opposite side ; the gneiss, however, there assumes so granite-like an 

 appearance, that the dip and strike are scarcely distinguishable ; and 

 this is often the case in this country. Indeed I find it a matter of 

 great practical difficulty often, to say whether a rock is granite or 

 gneiss; therefore I call these rocks *' gneiss-like granite" or *' granite- 

 like gneiss," according to their appearance. 



Hypothesis of origin of the metalliferous condition of the rocks. — 

 As to the nature of the process by which the deposition of metal in 

 these axes has taken place, I must leave the solution of the question 

 to abler and more experienced heads than mine. I can only offer 



* The formation of a vein of the sulphuret may be illustrated by specimen 

 No. 80 ; the gradual infiltration of small masses of the same material, by speci- 

 mens 79, 82-85, and the completion of the process by the removal of the granite 

 entirely, and its displacement by pyrites. (See specimens 86 and 81.) Other 

 specimens will illustrate the same process with the oxides and silicates, where 

 the felspar will in some be seen just tinged with the contact of the mineral, but 

 only slightly, if at all decomposed. Next, the felspar has lost its hardness, but 

 still retains its crystalline form. Again, the constituents of the original rock 

 have almost disappeared, but the change is known by the felspathic portion now 

 decomposed into clay, coloured by oxide or silicate, adhering to the tongue. 

 Nos. 51, 57, and 88-91 will, I think, show these changes. 



f Perhaps I ought to apologize for the use of this expression ; but I apply it to 

 those axes which do not permanently change the dip of the gneiss, in contra- 

 distinction to the three I have mentioned above, on either side of which the dip 

 continues nearly the same for miles. These last I would call " axes of change." 

 Some of the former kind of axes present very little metallic appearance in parts 

 of their course, but, I think, are traced for a great distance (which they often 

 may be, from the two sides formed by the almost vertical rock being infiltrated by 

 quartz, which renders them conspicuous). Most of them will be found to contain 

 iron, copper, or chrome in some parts. A mine was worked in one place in a 

 green stain of chrome which was mistaken for copper. 



