OF ATACAMA AND OOQUIMBO. 287 



that which contains the richest mines of Arqueros, is only euritic rock, compact, gray, green- 

 ish, or iv.ldUli, ami in which thd-,- are no traces of crystals. 



"Whether compart, mritic, porphyritic, or lim-cia-form, in general all these rocks efTervesce 

 more or less with acids, and are impregnated with carbonate of lime, manganese, and iron. A 

 specimen from the enclosing rocks of the richest vein gave twenty per cent, of these minerals. 



"There is only a small number of mineral species in the felspathic mass I have just described. 

 Above all, we remark the absence of quartz and mica accompanying auriferous deposits and 

 iiniphihole, more extensively spread than any species in the system of the Andes, and which 

 ordinarily is found with copper mines. The most abundant mineral that which constitutes 

 the gangue of the ores, and forms an infinity of veins, lines, and nuggets through the entire 

 extent of the mountain is sulphate of barytes, associated with spathic lime and carbonate 

 of barytes. This substance also serves as a guide to miners in their search for ores. It is 

 almost always crystalline and lamellar, but never crystallized or transparent. Fissures five to 

 six feet broad are found filled with this mineral, in a nearly pure state, containing no traces of 

 silver. When such traces are found, it often becomes compact and mixed with ferriferous, cal- 

 careous, or cobaltiferous portions." 



There are two principal veins parallel with and distant from each other, about half the 

 diameter of the hill. They crop out 120 feet below the summit, extend for a mile and a half 

 along a northwest and southeast line, and descend almost vertically, having only a slight dip to 

 the southwest. The richest of the two veins, and whose discovery was due to the silver stones 

 found in the ravine, is from two to three feet wide, sometimes separated into several strings, 

 none of which ever depart far from the general line of the principal fissure. Nearly all of the 

 best ores have been obtained from within about 150 horizontal feet of the central portion, after the 

 vein had been wrought to a depth of sixty feet. Silver disappeared entirely at the depth of 

 230 feet. Neither of the veins had been worked 300 vertical feet ; and both Dr. Darwin and 

 Prof. Domeyko describe them as in bad condition, owing to the want of proper machinery 

 to keep them free from water, and loss of the richest ores by theft. 



The principal and almost the only valuable ore, and that which renders the mines remarkable, 

 is a hydrargyrite of silver, or natural amalgam, whose composition is atomic and constant. 

 Prof. Domeyko thinks it has not been found in any mine of silver or mercury of the old or 

 new continent. His analyses gave, 



Silver .... 0.865 

 Mercury . . . 0.136=1.000 



or two atoms of silver for thirteen of mercury ; that of Prof. Smith will be found in Appendix D. 



Below the arquerite, as this mineral has been called, small quantities of filiform silver have 

 been found. Cobaltiferous sulphuret of silver is often obtained with the rich ores from the centre 

 of the vein ; arseniate of cobalt, as in nearly all the silver mines of Chile, is a constant com- 

 panion ; and multiple cupreous sulphurets are quite numerous near the lowest wrought portions 

 of the fissure. Besides these, there have also been found greenish arseniate of nickel, galena, 

 and iron pyrites. 



In the secondary stratified porphyry of the same meridian as Arqueros, and about thirty miles 

 south of it, is Los Algodones, a mine not less interesting to the mineralogist. The height 

 of the hill in which it is contained is rather less than that of Agua Amarga, and the veins 

 are near its summit. Their general direction is north 5 to 7 west, the dip very slightly 

 to the east, and their thickness from two to three inches. The enclosing rock is porphyry : 

 sometimes of a sombre violet-brown, spotted with many grey, green, and bluish shades ; and 

 sometimes of an ashy grey spotted with red. Occasionally it is found brecciaform ; that is to 

 say, the spots resemble the angular fragments of breccia. The structure of the walls is almost 

 always earthy, and rarely compact ; their felspar crystals very small and irregular, as in most 

 of the stratified porphyries ; and in the neighborhood of the fissure we find calcareous spar, 

 disseminated in nodules, filets, and very irregular veins. 



