J. H. COLLIXS ON THE GEOLOGY OP THE RIO-TIITTO MUTES. 245 



26. On the Geologx of the Rio-Tinto Mixes, luith some Geisteral 

 Ee3IAR]is on the Pyritic Region of the Sierra Morena. By 

 J. H. Collins, Esq., F.G.S. (Read January 28, 1885.) 



[Plate VI.] 



1. Introduction. 



The pyrites deposits of the Andevallo, as the mineral zone of the 

 south-western branch of the Sierra Morena is called, are certainly 

 the most extensive ever yet discovered. In addition to the world- 

 famous mines of Eio Tintoand Tharsis in Spain, and of San Domingos 

 in Portugal, they include many others of very considerable extent 

 and importance, yielding in the aggregate, at the present time, up- 

 wards of two and a half millions of tons of ore per annum*. 



The deposits in question are included within a band of country 

 about 140 miles long and 30 miles wide, consisting for the most 

 part of Palaeozoic schists, now known to be principally, if not en- 

 tirely, of Upper Devonian age, but which are often very highly 

 metamorphosed locally into jasper, talc-schists, chiastolite-schists, &c., 

 and are associated with quartz- and felspar-porphyries, diabase, 

 quartz- syenite, and granite. The mineral strata to the northward 

 abut against or rest upon the highly crystalline schists and gneissose 

 rocks of the Sierra Alta, which have been assigned to the Huronian 

 and even to the Laurentian period, while to the southward they are 

 overlain by Tertiary limestones and sandstones. 



The chief mineral riches of this region consist of masses of cupri- 

 ferous pyrites, such as those so largely worked at the above-mentioned 

 mines ; but there are, in addition, numerous veins of manganese 

 ore, as well as of lead, copper, and zinc ores, some of which have 

 been worked occasionally to considerable advantage, while a very 

 large number of them could be so worked were the country more 

 opened up by railways and roads. 



2. General Character and Associations of the Pyrites Deposits. 



Commencing near Aznalcollar and Castillo de las Guardas, in the 

 province of Seville, the masses of pyrites succeed each other at short 

 intervals, proceeding westward through the middle of the province 

 of Huelva to San Domingos in Portugal t. The more important 

 masses are, however, contained within a much more circumscribed 

 zone, which is not more than three or four miles wide at the eastern 



* The official statement in the ' Eeyista Minera' gives 1,720,853 tons as the 

 total production of the Spanish deposits in 1882, since wfiich there has been a 

 very considerable increase. In 1884 the mines of Eio Tinto alone produced 

 1,369,918 tons of ore, containing on an average 3-234 per cent, of copper. 



t This region, besides supplying numerous tributaries to the rivers Gruadal- 

 quivir and G-uadiana, is the source of the considerable river Odiel, which, with 

 its chief tributary the Tinto, enters the sea at Huelva. 



Q.J.G.S. No.163. T 



