250 J. H. coLLi]srs on the geology 



traversed by mineral veins ^delding ores of lead and copper, bnt of 

 quite a different character from those which yield the pyrites. The 

 soil in the syenite regions is richer than in those composed of por- 

 phyry or slate. Further to the north the syenite is succeeded by 

 hornblende-schists, and then again by limestones. I reserve a fuller 

 description of these rocks, which nowhere approach within a league 

 of the mines, for a future paper. 



Diabase. — The chief development of diabase is to the south of the 

 mines, where it occurs in extensive masses, in part intercalated 

 mth, and in part traversing, the slates. The general strike of these 

 basic rocks is a little to the south of west, and associated with them 

 are others of a fragmentary nature which have much the appearance 

 of altered ash-beds. 



The diabase beds are readily discoverable wherever they occur by 

 theii^ brown colour where decomposed, and by their almost universal 

 tendency to spheroidal decomposition. This, in some parts, is so 

 marked that the mule-tracks which serve as roads are strewn with 

 rounded masses of rock varying from a few inches to a foot in 

 diameter. These compel the attention of the traveller, even when 

 not a geologist, owing to the extreme inconvenience which they 

 occasion, especially if he should happen to be a pedestrian. As 

 stated above, rocks of a very similar character are associated with 

 the quartz-syenites which lie more to the north. 



The decomposition of this rock produces a very rich soil, which 

 may be distinguished at once by its vegetation from that of the 

 slates and the porphyries, the latter being extremely barren. 



The following analysis (D) will fully account for this richness of 

 the soil. The large proportion of " soluble silica" is noteworthy in 

 this analysis : — 



D. 

 Water by desiccation . . 0-50] .pp 



„ by ignition .... 446 J 

 Silica insol. in HCl. . . 47' 70 \ .q on 



Silica sol. in HCl 1-50 J ^^^ ^ 



Alumina • 17'26 



Protoxide of iron (with a little "1 1 9.94 ~ 



peroxide) J 



Lime 6*94 



Magnesia 4*74 



Soda (with a little potash) . . 3-41 



Loss l"5o 



100-00 

 Specific gravity 2*931 

 The Porpliyries. — These are for the most part quartz-porphyries, 

 which have nothing particularly remarkable in their appearance 

 when compared with similar rocks from other districts. Some are 

 more highly quartzose than others, but in all porphyriticaUy developed 

 crystals of felspar of more than microscopic size are extremely rare 

 except in a few localities. Lithologically two varieties are di- 



