OP THE EIO-TINTO MINES, 251 



stinguishable, compact and schistose, but passing into each other. 

 In general the interior of a mass is compact, while a schistose 

 structure becomes more and more evident as the outer portions 

 are reached, i. e. those near the junctions with the slate. The 

 difference in chemical composition between the two varieties of 

 the porphyry is by no means great ; and I am disposed to believe 

 that the schistose structure has been developed by a continuance 

 of the same pressures which have converted the Palaeozoic mud- 

 stones into slates. Eeference has already been made to the existence 

 of bands of porphyritic schists (altered shaly sandstone) in the 

 slates. These so closely resemble the bands of schistose porphyries, 

 that it is not in all cases possible to distinguish between them 

 in hand specimens ; indeed, the more one examines these rocks 

 in the field, the more one is disposed to regard them merely as 

 varieties of one and the same rock, serving as intermediaries in 

 bridging over the passage from undoubted slates on the one hand to 

 undoubted felspar-porphyries on the other. 



The following analyses of various specimens of the porphyries 

 will show how closely they resemble each other. I add an analysis 

 of the porphyritic schist for comparison : — 



E. F. G. 



Porphyritic Schistose Yery solid 



schist. porphyry. porphyry. 

 Water over H,SO. by 0-10 ) ^.,^ 0-25 \ 0-12 1 ^.^^ 



„ Ignition U'dO J 0'4o j 0-z» J 



Silica 64-50 67-00 76-34 



Alumina 23-60 20-30 14-85 



Oxide of iron 2-16 2-88 1-89 



Lime 1-40 2-80 0-10 



Magnesia 0-40 trace 0*50 



Pyrites trace trace trace 



Phosphoric acid 0-30 trace 0-05 



Alkali as potash 3*20 2-10 5-11 



Carbonic acid, fluorine, 



and loss 4-04 4-22 0-76 



100-00 100-00 100-00 



Specific Gravity ,. 2-62 2-60 2-65 



Occasionally, as at the old-station quarry at E,io Tinto, the 

 porphyritic schist may be traced right up to its contact with the 

 compact porphyry, becoming more distinctly crystalline as it ap- 

 proaches the junction, and leaving no doubt in the mind of the 

 observer that the intrusive porphyry has been an active agent in 

 producing the metamorphosis. 



It is remarkable how uniform in appearance the porphyries are, 

 even when taken from localities widely separated from each other. 

 Thus I have specimens from Yalverde on the south, which cannot be 

 distinguished from others broken near Madrono on the east, and 

 Poderosa in the north. 



The porphyritic masses are usually lenticular in form, but they 



