OF THE EIO-TINTO MINES. 247 



resembling the G. crenistria of Phillips, an Avicula, an Ortlioceras, 

 and two specimens of Crossopodia ; but these were not found within 

 the area included in my map, although they are stated to have all 

 occurred associated with Posidonomya Becheri*. Many of these 

 fossils were found enclosed in concretionary nodules. 



Up to the present time fossils have been discovered in eight 

 different localities, all situated within half a league of the village of 

 Rio Tinto, and all within a furlong of the mineral deposits f. 



For the most part the slates are either vertical or very highly in- ■ 

 clined (see Plate VI. tig. 2) ; and with a few local exceptions the dip 

 for miles together is northward, while the strike is from 15° to 25° north 

 of west. At the surface the slates are mostly weathered to a yellowish 

 or brownish tint, with occasional bands nearly black ; but in almost 

 all the excavations a dark-blue tinted rock is soon reached, more 

 or less approximating in character and appearance to roofing-slate. 

 Up to the present time, however, no slate has' been met with of 

 marketable value. 



The following are analyses made in illustration of this paper : — 



(A) is of a specimen taken from an adit-level driven recently by 

 the Ayuntamiento of Rio Tinto into the hill known as the Mesa de 

 los Pinos for the sake of obtaining a supply of drinking water, the 

 place selected being, therefore, situated at a considerable distance 

 from the pyrites deposit. The specimen selected was very dark blue 

 and exceedingly close and compact in texture. 



(B) is a sample of weathered rock, originally of very similar 

 character. 



A. B. 



Water by desiccation 0-26 1 . ^o 0-20 \ ^ 



„ by ignition 4-4:6 j ^ '^ 2-50 J ^* 



Silica 61-12 64-47 



Alumina 18-95 19-22 



Protoxide of iron 9-65 trace. 



Peroxide of iron trace. 9*20 



Lime . 0-49 trace. 



Magnesia 0-72 trace. 



Pyrites 0-30 0-10 



Alkalies, calc. as potash . . 2-19 3-21 



Copper trace. trace. 



Phosphoric acid 0-12 trace. 



Loss 1-74 1-10 



70 



100-00 100-00 



Sp. gr. 2-73 Sp. gr. 2*71 



* As ilhistrating the amount of extension which a shell may undergo from 

 lateral pressure, I may here call attention to a statement of the author above 

 quoted that he has in his possession specimens of P. Becheri, 160 millimetres 

 (say 6 inches) long. 



i" The first fossils in the slates were, I believe, found by Mr. Osborne, now 

 the deputy-manager of the Rio-Tinto mines, at the point marked A on the map, 

 while making the railway leading to the Cerro pump. I have found many 

 others at B which seem to be from the same beds, arched over as shown in the 

 section. 



t2 



