136 Mr. D. Porbes's Researches on the 



apparently are the representatives of the European "Kupfer- 

 schiefer. - " 



In the mine of Buen Pastor at Corocoro it is found in the 

 form of irregular nodules imbedded in the sandstone along with 

 metallic copper and minute grains of metallic silver i the mine 

 itself is explored for silver as well as copper. 



The nodules vary much in size, from that of a pea up to some 

 three or four inches across, irregularly imbedded in the compa- 

 ratively soft red sandstone, which in the proximity of the nodules 

 appears deprived of its red colour, or, as it were, bleached, appa- 

 rently by some deoxidating agency — a phenomenon which is also 

 the case in such parts of the sandstone bed in which metallic 

 copper or silver is found, and thus serves as a guide to the miner 

 in rejecting the non-metalliferous or unaltered part of the rock. 



On fracture these nodules have considerable glance, and at first 

 sight appear to be nearly pure Domeykite ; but upon closer ex- 

 amination they are seen to consist of an intimate mixture of that 

 mineral with quartz grains, the metallic glance of the Domeykite 

 obscuring the colourless particles of sand. 



The specific gravity of the richest part of such nodules was 

 found to be 4"65 at 60° F. (15°-5 C), whilst the metallic powder 

 obtained upon pulverizing the nodules and washing out the in- 

 termixed quartz was found to be 6*91 . 



An analysis was made of these metallic particles, and the 

 results obtained showed the mineral to have the following per- 

 centage composition : — 



Arsenic . . . . 28*41 

 Copper .... 71-13 

 Silver Q-46 



100-00 

 Considering the equivalent of copper to be 32, and that of 

 arsenic 75, the formula Cu 6 'As, as accepted for Domeykite, 

 w r ould by calculation require* — 



Arsenic .... 28*19 

 Copper .... 71-81 



10000 



Titaniferous Iron-sand (Kibdelophan ?) . — Along the west coast 

 of Peru at Arica, the sands which cover the barren porphyries 

 and other rocks subordinate to the upper oolitic series contain a 

 considerable intermixture of black particles, easily extracted from 

 the bulk of the sand by drawing a magnet through them, to 

 which these particles adhere. 



Although they appear homogeneous to the eye, a microscopic 

 examination shows them to be rounded irregular grains, w r hich 

 still retain some adherent gangue, colourless,or of a reddish-brown 



