248 J. H. coLixN's oy the geology 



It ynH be seen from these analyses that the result of weathering 

 is to reduce the amount of combined moisture, to dissolve out the 

 lime and magnesia, and to peroxidize the iron. The alkalies do not 

 appear to have been removed in the process of Treathering ; in fact 

 the second shows somewhat more than the first. All this is e.xactly 

 what might have been expected of such a rock, in such a climate, 

 ■where considerable alternations of temperature are continually 

 occurring, while there is ordinarily but little change in the hygro- 

 metiic condition of the atmosphere. 



Eeference has been made above to the prevalence of a northern 

 dip at a high angle. In a distance of six miles from Is", to S. I 

 have only discovered as yet three anticlinal curves, and this not- 

 withstanding the fact that the bare rocks are everywhere visible. 

 These are shown on the accompanying enlarged cross-section of part 

 of the region shown on the map (Plate \1. fig. 2). It would appear, 

 therefore, that these ancient slates have a very great thickness here as 

 in many other places. The three anticlines referred to may be seen 

 at the Eejondillo near the new bridge, in the valley of the Kio Agrio, 

 and on the Eio-Tinto railway between Xaya and Jaramar. In many 

 places the slates are more or less sandy in appearance; they pass, in- 

 deed, into a grey fissile sandstone. The development in coarse-grained 

 varieties of these rocks of crystalline particles of quartz and felspar, 

 the latter now for the most part somewhat kaolinized, results in the 

 formation of a rock scarcely distinguishable, except by the use of a 

 lens, from the more schistose varieties of the porphyries hereafter 

 to be described. 



In seme places the slates enclose fragments of preexisting rocks, 

 rounded or angular. In others small lumps, bands, or lenticular 

 masses of cpartzite occur imbedded in the slates, having all the 

 appearance of foreign enclosures, but which I have reason to believe, 

 in some instances, are merely portions of the original rock of more 

 sandy texture than usual, which have become silicified by aqueous 

 agency, the siliceous waters having found in them fit situations in 

 which to deposit their silica. Subsequently the contemporary quartz- 

 ites have become traversed by joints which do not enter the main 

 mass of the slate, so that the silicified portions completely simulate 

 foreign enclosures. These phenomena may be well seen at several 

 points on the road from Zalamea to Talverde ; and they are par- 

 ticularly well marked on the hiU between Zalamea and the halfway 

 house on the road to Eio Tinto. Here great masses of this quartz- 

 ite, some of them several yards in extent and many tons in weight, 

 occur in slate, having all the appearance of included fragments, but 

 which, I believe, have been formed as stated above. 



In the neighbourhood of the pyrites deposits the schists are often 

 highly charged with pyrites, as might be expected. I believe, 

 indeed, that they have supplied the material for those deposits, but 

 this part of the subject will be reverted to hereafter. 



The pyritous schists near the surface are sometimes changed into 

 ferruginous schists by the oxidation of the pyrites. More often, 

 however, the silicified ferruginous bands which indicate the presence 



