OF THE EIO-TLNTO illlfES. 249 



of pjTitous deposits, appear to have been produced hy infiltration of 

 mioeral solutions from lakes of ferruginous water which formerly 

 existed. The effect of this infiltration has been to indurate the 

 nearly vertical schists in such a manner that masses of them have 

 resisted denudation, and now form ridges liigher than the surround- 

 ing country. In the neighbourhood of the fissures here, as else- 

 where (Agordo, &c.), both schists and porphyries are often bleached 

 and kaolinized, 



Jasper, — In a great many places fine-grained bands of the slates 

 following the strike are found to be silicified, and converted more or 

 less completely into excellent jasper, mostly red, but sometimes of a 

 deep green tint. The transition from the unaltered slate to the jasper 

 is often abrupt, but occasionally very gradual, and specimens may 

 easily be obtained in all stages of alteration. The production of this 

 jasper does not ax^pear to be necessarily connected with the presence of 

 intrusive rocks at the surface, since some well-marked bands are 

 situated at a considerable distance from such rocks. At the same 

 time it must be admitted that the finest and most extensive bands 

 are situated very near the porphyries. In many cases these jasper 

 bands accompany bands, nodules, or " eyes" of very pure peroxide 

 of manganese, to which further reference will be made hereafter. 



The following (C) is an analysis of one of these red jasper bands 

 from near Bella Yista : — 



C. 



Silica 90-30 



Peroxide of iron 6-00 



Alumina 1*30 



Lime '30 



Magnesia '10 



Manganese trace. 



Alkalies - -30 



Water -40 



Loss 1-30 



100-00 



Assuming that the original slate had pretty much the composition 

 of analysis A, a comparison of this analysis with C will show 

 how great the change has been. It is not merely that there is a 

 great addition of silica, but almost the whole of the other con- 

 stituents have been carried off except the oxide of iron, and the 

 ratio of that to the silica has been reduced from ^ to ^-. 



Syenite. — A few miles to the north of the mines is a broad band 

 of syenite running nearly in the same direction as the slates, which 

 I believe to be more ancient than any other of the eruptive rocks of 

 the district. In the neighbourhood of Campo Frio it has a breadth 

 of nearly four miles. It is a perfectly typical quartz syenite, and 

 is itself penetrated by veins and masses of diabase, often much 

 decomposed, and by dykes of quartz-porx^hyry and of felsite, much 

 resembling the more compact of the elvans of Cornwall. It is also 



