246 J. H. coLLZN-s o:n the geology 



extremity, but whicli divides into two branches in going westward, 

 the smaller and less important trending a little to the north of west 

 towards Aroche, while the greater passes to the south of west into 

 Portugal at San Domingos. 



I do not propose to describe this extensive region here *, but the 

 description which follows of the Eio-Tinto district, and of the 

 relations of its mineral deposits to the country rocks, will apply 

 with very considerable accuracy to the whole region, allowance 

 being made for the greatly different extent of the various deposits. 

 The more important mineral masses throughout the region are very 

 frequently true " contact-deposits," and I believe the more closely 

 they are examined the more generally this will be found the case. 



3. General Descrijption of the JRio-Tinto district. 



Briefly the structure of this district may be described as under : — 

 The greater portion of the countrj^ represented on the accompanjing 

 map (Plate YI.) consists of Palseozoic slates and schists, striking some 

 few degrees to the north of west, and dipping, for the most part, 

 steejDly to the northward. These slates have been pierced by in- 

 trusions of syenite, of diabase or dolerite, and of quartz-porphyry 

 and felspar-porphyry. Fissures have then been opened in these 

 rocks along lines of weakness, mostly determined by the junctions 

 of the porphyries with the slates, and these fissures have become 

 filled in their wider portions wdth rock debris or with pyrites. The 

 section fig. 2, Plate YL, shows the manner in which the various 

 rocks are convoluted and interstratified. 



The Slates. — At and around the mines of Eio Tinto these are 

 xiertainly of Palseozoic age, but their exact geological horizon has only 

 lately been determined. At one time they were stated to be entirely 

 unfossiliferous ; then they were said to be Silurian f. Messrs. Pomer 

 and Wimmer consider them to be the equivalents of the culm- 

 measures J ; while Dr. Fraas, of Stuttgart, on the occasion of his 

 visit to the mines in 1883, came to the conclusion that they were of 

 Upper Devonian age, and this conclusion is fully borne out by the 

 examination which Dr. Etheridge has been kind enough to make of 

 the fossils which I obtained and submitted to him. Fossils are 

 tolerably abundant in several places in close proximity to the mineral 

 deposits. As might have been expected, they are usually much 

 crushed and distorted, yet it has been possible to determine several 

 species, the most common being Posidonomya Becheri, P. acuticosta, 

 P. lateralis (?), a Goniatite allied to G. suhsulcatus, and a j)lant 

 which may be a Sagenaria. Don Joaquin Tarin, in his memoir 

 published in 1878, states that he has also met with a little Goniatite 



* The main portion of this region, that comprised within the proyince of 

 Huelya, has been very fully and faithfully described by Sr. Dn. Joaquin Gron- 

 zalo y Tarin in the ' Boletino de la Comision del Mapa Geologica de Espana,' 

 vol. V. 1878. This author gives also a very good geological map of the Ande- 

 Tallo on a scale of ^owoo- 



t Quart. Journ. Geol. Soc. vol. xxxvii. p. 2. 



X Berg. u. Hutt. Zeit. J^os. 28-30, 1883. 



