254 



J. H. coliIjS's o:^ the geology 



Above sea-level. Above sea-level. 



-^ r Cerro Colorado. . 1794 ft. 4. Eio Tintillo . . 1370 ft. 



* \ Cerro Salomon . . 1761 „ ^ J Los Planes . . 1200 



■I 



2 r :y:esa de los Pinos 1500 „ \ El Lago .... 1189 „ 



' \ Cerro de las Yacas 1497 ,. 6. La Z^ava .... 1050 „ 



o J BeUa Yista 1473 „ 



• \ Atalaya 1470 „ 



IS'otwithstanding the geologically recent origin of these ferrugi- 

 nous lake-deposits, they are often traversed by quartz -veins filling 

 shrinkage- cracks, and occasionally small masses of opal (Miiller's 

 glass) occur in cavities, showing that there must have been circula- 

 tion of siliceous waters and deposition of silica in geologically 

 recent times in this district. 



In addition to the iron-ore of these hill-top deposits, a very large 

 quantity of excellent iron-ore of precisely similar character has been 

 got out of the " over burden" recently removed from the north side 

 of the great open cast on the South lode. This has, I believe, been 

 formed by the degradation of the ancient bog-iron deposits, and 

 has sunk down over the pyrites into the extensive pre-Roman 

 open workings. These are filled with brecciated masses, consisting 

 largely of fragments of the iron-ore, mixed with slate and porphyry 

 frequently in a state of complete decomposition. 



Occasionally the pyrites has decomposed near the surface so as to 

 form a true "■ gozzan " extending downwards for many fathoms, thus 

 producing a ferruginous substance capable of being used as iron-ore 

 under favourable circumstances, although, as it contains a large ad- 

 mixture of clayey and earthy matters from the sides of the lodes, 

 it is not usually preserved as such. Samples of this ferruginous 

 earthy matter often contain from 35 to 45 per cent, of metallic 

 iron, as shown by analysis. 



It is interesting to note that the waters issuing from the mines 

 even now deposit considerable quantities of oxides of iron in the 

 beds of the rivers wherever there are convenient situations. The 

 following are analyses of some of these substances ; (H) — yellow 

 ochre from the bed of the Eio Tinto near !^anantiales, about 20 miles 

 below the mines ; (I) — iron-ore from the Eio Tintillo : — 



H. I. 



SiUca and insoluble .. 25-00 13-64 



Alumina 2-50 1-06 



Perric sulphate 9-00 



Perric oxide .... (46'80)=hydrate 57-78 . 73-00 



Arsenic 0-27 



Copper . trace. trace. 



Moisture and loss .... 5-45 12-30 



100-00 100-00 



The following may be regarded as t}-pical analyses, (J) of the 



