OF THE RIO-TIXTO MINES. 263 



(71) The formation of rich veinlets or " leaders " of ore Trithin the 

 masses has been the result of subsequent operations, probably at very 

 many different times. These veins appear to occupy faults and 

 shrinkage-cracks, and to have been gradually filled by a segregation 

 of substances from the main masses of pyrites. 



(0) Abundant evidence of numerous movements Tvithin the masses 

 of pyrites is afforded by the numerous slickensides which are every- 

 where and continually met with. 



(/:>) The formation of the ironstone-beds which now cap the Mesa 

 de los Pinos and other hills, although immeasurably more recent than 

 that of the pyrites-deposits, took place so long ago that deep valleys 

 have since been excavated through the iron-ore ; and the general 

 level of the country has been so altered that the aucient lake-deposits 

 now exist as hill-tops. 



3. As to the Surface-Geology : — 



The broad features of the geology of the district, which have been 

 described above, are very evident to any one who has studied the 

 relations between geological structure and scenery. The slate- 

 regions, which for the most part form the lower lands, generally 

 consist of a series of low hummocky hills of a very bare and unin- 

 teresting aspect; but occasionally the more silicified bands rise 

 into sharp ridges of a rugged appearance, which, in the transverse 

 valleys, are sometimes extremely picturesque, as in the gorge leading 

 up to the Campo Frio " digue " *. The jasper- and manganese-bands, 

 too, are generally evident as distinctly purple stripes in the otherwise 

 dull lead-coloured country. The barrenness of the slates seems to be 

 due to their vertical condition, to the great and long-continued heats 

 of summer, and to the absence of springs. The vegetation, where 

 there is any, consists generally of the dull dark-green foliage of the 

 gum-cistus. The valleys and watercourses, on the contrary, where 

 the effects of the long summer drought are less felt, are usually filled 

 with oleander bushes, whose abundant pinkish-red flowers mark out 

 the topography in a very striking and beautiful manner. The por- 

 phyries are scarcely more clothed with verdure than the slates, but 

 the hiRs are much higher and more irregular in form, and the ground 

 is generally strewn with rough greyish fragments of the rock. The 

 asphodel, a palmetto, and little scrubby acacias are more frequently 

 to be seen in the porphyry -tracts. The diabase has much richer 

 vegetation, with many olives, oaks, and cork-trees, the surface 

 between the trees being covered with brown and rounded fragments 

 of decomposing diabase, while the rock-exposures are often markedly 

 spheroidal or even columnar. The syenite has a very similar vege- 

 tation to tlie diabase ; but the country rock is greyer, and the hills 

 crowned with loose rocks, which often remind one of the granite tors 



'^^ At Kio Tinto a reservoir of water, shut into a natural hollow by an arti- 

 ficial wall or bank, is called a " Digue " or a " Dam " indiscriminately, 'although, 

 of course, the term is only properly to be applied to the enclosing masonry or 

 earthwork. 



tj2 



