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



the magnetic needle. At the spot where the experiments were tried, 

 the rock was granite or gneiss, with very httie appearance of iron ; 

 but still a detached piece of it at one spot affected the needle. I 

 will further examine this point *. 



The yield of the mines. — As to the prognosis of these mines, I 

 cannot at present regard it as favourable. This mine (Springbok) 

 continues, at the greatest depth yet worked, to yield abundant nests 

 and small veins of good oxide and sulphurets ; but the depth does 

 not exceed 40 feet at the most. 



At Concordia the proportion of stone to the pyrites is very great 

 at the same depth ; and the rock seems likely to be a hard non- 

 metallic felspathic granite at a few fathoms deeper. 



At Schaap River Mine, about four miles E. of Spectakel, a level, 

 driven through the base of the hill at right angles to the course of 

 the ore, cuts through hard and unstained granite-rock. Indeed, 

 from the evident connexion of these deposits with the action of 

 water, and the slight depth to which the hardness of the rock enables 

 us to penetrate, I think it would be unreasonable to look for rich 

 deposits at any great depth. 



Northern district. — Very different from those which I have de- 

 scribed are the deposits of ore in the vicinity of Henkries, and 

 thence ascending the Orange River to Pella. The rocks of this 

 part of the country are principally mica-schist, hornblendic schist, 

 and chloritic and felspathic rocks, with occasional large beds of 

 gneiss. These are penetrated by granite-dykes in every direction ; 

 many taking the line of strike, others cutting the strata at all angles. 

 I cannot help thinking that the close study of the rocks of this 

 country will greatly modify the opinions of geologists as to the 

 relation between granite and gneiss and other metam.orphic rocks. 

 I have seen a granite-vein of a serpentine form, not 1^ inch wide, 

 and less than 2 feet in length ; the gneiss through which it passed 

 was not at all disturbed. Again, I have seen in gneiss apparently 

 isolated masses of granite, from the size of one's hand to that of 

 a mass of many tons weight. Many such things as these seem 

 to me to throw doubt on the question, whether granite always pene- 

 trated the strata from below. 



Occurrence of ore in the northern district. — I have said that 

 the Orange River country is composed of hornblendic, micaceous, 

 and other rocks, and is penetrated by granitic dykes and masses. 

 Oxides, sulphurets, silicates, and carbonates of copper occur in small 

 masses, disseminated through the crystalline veins ; and sometimes 

 in the rocks or their margins. The veins which contain the most 



* Since I finished this letter, I have made experiments on the magnetic phae- 

 nomena of Springbok Mine, and I have satisfied myself that there was a consi- 

 derable effect exerted on the needle. Indeed it was nearly deflected to an angle 

 of 90° from its proper direction in a recent cutting in the drift-work of the upper 

 part of the mine. Mr. Green assisted me in these experiments, which I endea- 

 voured to free as much as possible from all source of error by trying parts of 

 the wall, detached, on the needle. It was not affected by either the rock or the 

 ores of the wall. 



