9'2 Reports and Proceedings. 



Has been extracted. There are also shafts, and, at some little dis- 

 tance, slag-heaps, where the ores have been smelted. The Buitron 

 deposit is a wedge-shaped mass, nearly cut in two by barren ground 

 in the middle. It slightly narrows in descending. The contents are 

 a nearly uniform grey-green ore of copper pyrites, crossed by bands, 

 •varying in the percentage of copper. The ore contains about 48 per 

 cent, of sulphur, from 10 to 20 per cent, of iron, from 1 to 5 per 

 cent, of copper, and some silica in a finely divided condition. It is 

 either sent to England as it is dug out of the mine, or treated by 

 cementation, as it is called — burnt by its own sulphur in the open 

 air, and the burnt ore treated for the copper. The precipitates are 

 also obtained by the usual processes. The lead is also saved, but is 

 inconsiderable. Another process is sometimes used (though not at 

 Buitron), suitable only to ores containing under 2 per cent, of 

 copper, called kernel roasting, whereby the copper is by very slow 

 combustion driven to the centre. The Buitron mass may be classed 

 with the great deposits on the same parallel, the Eio Tinto, the 

 Tharsis, and the San Domingo. These masses of pyrites are from 

 50 to 1200 fathoms long, and from 3 to 50 fathoms broad between 

 the walls. The mass of mineral in the vein has sunk, owing to the 

 process of oxidation by air and water, leaving the walls standing out 

 above the si;rface in a most remarkable manner. The wash from the 

 sm-face of these masses deposits hydrated oxide of iron. The whole 

 range of the deposits, occasionally metalliferous in irregular portions, 

 is upwards of 140 miles, the lode vdnds a little in a limited band of 

 country running nearly east and west. The mineral is essentially 

 the same, but richest in copper at Buitron, near the disturbed edge or 

 side. The coarse schistose rock is not much metamorphosed, except 

 in certain places near the greenstones. The successions are very 

 thinly bedded. In none of these rocks does there appear to be 

 any true slaty cleavage, though there are occasionally good slate 

 flagstones. The literature of this subject may be found in Mr. 

 Mason's original pamphlet on the Mines of Huelva ; in one or two 

 Spanish reports ; Mr. J. L. Thomas's pamphlet on the Eio Tinto 

 Mines ; and Mr. G-reen's able description in the Quarterly Journal 

 of Science for October, 1868. — The great commercial importance of 

 these deposits arises from the demand in Great Britain for these 

 ores for their contained sulphur. They are used at Newcastle and 

 elsewhere in the manufacture of sulphuric acid for artificial manure. 

 The trade is of considerable dimensions, amounting altogether to 

 the raising and exporting, from all the mines, of upwards of 500 

 tons a day, and for this three railways have been constructed. 



coI^I^ESI=OIs^3DE3^CE]. 



"PHOLAS" HOLES IN LIMESTONE. 

 Sir, — T am unwilling to let the month pass without acknow- 

 ledging the weight of the Eev. T. G. Bonney's observations (on pp. 

 483, etc.) on certain notes of mine on some superficial phenomena at 



