18G 



ON TlIK 'J'(Jl'<)(,'lIAl'Hr AND GEOLOGY 



the auriferous earth is ciearly derived from the decomposition of the underlying* 

 slates ; and hnally gold is nowhere found in those parts of the streams running only 

 in the eruptive rocks, unless a helt of slate crosses still higher up. Nor is it ever 

 found in the earth overlying the syenites. Throughout this region syenitic dykes 

 are constantly encountered, and the upper half of the Jivanii runs through the parent 

 mass, while veins of the same rock ci'op out in several places in lower part of its 

 course as well as at the mouth of the Anones, and in the bed of the neighboring part 

 of the Jaina. We have thus a further proof of the theory already enunciated* that 

 the proximity of eruptive masses, is the cause of the presence of gold in the quartz 

 veins of this country, . . . ■ - , 



On the Jivana there is a more than usual variety in the appearance of the syenite. 

 In every place I found it flesh coloi'cd owing to a pinkish feldspar ; in another it is 

 dark gray and very fine-grained ; and in still another it is white with acicular crys- 

 tals of hornblende. ]Srear there in the Jaina near the mouth of the Anones, there is 

 a little dyke made up of white quartz, white and pink feldspar and with little isolated 

 g'rains of a black mineral which I could not determine with certainty ; and between 

 the Anones and the river is another in which the materials are very unevenly dis- 

 tributed, hornblende occurring both in irregular masses and in isolated crystals. 



South of this region the quartz veins still occur in the slates and on the Susua 

 and Medina Creeks some gold is found, but the eruptive dykes here disappear and 

 the quantity of gold rapidly dimishes. It is said to be found as far sOuth as the 

 Cuallo but this requii'es corroboration. If it does occur there it is in very small 

 quantities. ... • , . .. ■ : . . , ^. 



This range of hills terminates to the southeast in a prominent point running out 

 into the savanas on the nearly flat summit of which is a little cluster of houses. 

 Here the rocks are more than usually metamorphosed, some of the beds being a black 

 mica slate in which occasionally there can be seen small white grains of quartz. 

 Throughout this rock are stains and an occasional little thread of copper ore. I dis- 

 covered green and blue carbonate, a little silicate and some purple sulphuret. ISTothing 

 approaching a vein structure has ever been detected, but Mr. Heneken, who seems to 

 have been possessed with a mania for copper mining commenced operations here, dug 

 innumerable little pits all over the hills and founded a settlement to which he gave the 

 name of Cobre. Since his time one or two other but less enei'getic attempts to de- 

 velop the " mines " have been made, but with no better success than he attained. He 

 also made similar essays at copper mining on the Arroyo de las Platanas, a little 

 tributary of the K'igua west of Cobre. Here he found more copper but no veins. 



* See pp. 89, 127. 



