OF THE EIO-TINTO MINES, 253 



below Gangosa, on the western prolongation of one of the branches 

 of the ISTorth lode. This fault-rock consists of a hardened felsitic 

 basis of disintegrated porphyry and slate enclosing angular fragments 

 or partially rounded masses of porphyry, slate, and occasionally 

 jasper — generally angular, and occasionally of considerable size. 

 The bearing of these facts upon the age of the faults is obvious 

 enough. 



Apparently the motion of the walls has broken up the porphyries 

 and slates, the latter having been previously metamorphosed into 

 the various forms noted above. In places the fissures must have 

 remained open for a considerable time, while large quantities of 

 fragments fell in and became surrounded by clayey matter. 



The Iron-ores. — In a considerable number of places, always on 

 hill-tops, and always near masses of pyrites of greater or less extent, 

 are horizontal beds of iron-ore, consisting of angular or occasionally 

 rounded fragments of quartz, slate, and other rocks, cemented 

 together by peroxide of iron, which is only slightly hydrated. 



The proportion of quartz or other matter in these rocks varies 

 from less than 1 per cent, up to 50 or 60 per cent. In a few places 

 these beds have yielded plant-remains, which indicate that they are 

 of Miocene age. It would appear that they were formed at the 

 bottom of lakes — the ferruginous matter having been derived from 

 the decomposition of the pyrites of the upper parts of the lodes — 

 which stood much higher than at present. These beds of ore occur 

 at various elevations, each different level no doubt indicating a 

 period of approximate constancy in ihe Jevel of the waters, the 

 different beds having been formed as the old lake-boundaries were 

 broken through, and new ones formed at a lower level*. 



The following patches of iron-ore of a sedimentary character 

 appear to indicate the former and successive existence in the neigh- 

 bourhood of the mines, of no new fewer than six ferruginous lakes. 

 The heights given are merely approximate, as determined by a 

 pocket-aneroid. The patches bracketed together appear to have been 

 once continuous, and have been separated by subsequent subaerial 

 denudation. 



* The chief of these deposits, that of the Mesa de Iob Pinos, has been well 

 described by Mr. J. A. Phillips in his paper " On the Occurrence of Eeraains of 

 Eecent Plants in brown Iron -ore," Q. J. Gr. S. vol. xxxvii. p. 1. He gives the 

 folloAving analysis of a sample of the iron ore from the Mesa, which may be 

 taken as a pretty fair average result for the ore generally exported, except that 

 there are usually present a few per cent, more of silica : — 



Water I ^^ygroi^etric 1-40 



^^'^^'^ 1 combined 11-85 



Silica 1-53 



Perric oxide 84*65 



Alumina trace. 



Phosphoric anhydride "14 



Sulphur '23 



99-80 



