262 



J, H. COLUXS ON THE &E010GY 



of the stratified beds into a rock resembling the more schistose 

 varieties of the porphyries ; but proofs of the distinctness in 

 origin of the porphyritic schists and the schistose porphyries are 

 abundant. 



(/) The conversion of certain bands of the slates into chert or 

 jasper was anterior to the appearance of the porphyries, since the 

 cherty bands are traversed by them similarly to the slates them- 

 selves, and since the " fault rock '' contains fragments of the former 

 enclosed Tdthin its substance. 



(g) As the porphyries were intruded into the stratified rocks long 

 after these had been tilted up into their present position (i. e. nearly 

 vertical), and as their composition, on the whole, is so similar, it seems 

 not unlikely that they are actually composed of the melted-up lower 

 portions of the synclinal curves. 



(Ji) "While the porphyries were still in a semi-pasty condition, the 

 pressures in directions normal to the strike probably continued to 

 act, thus producing their markedly schistose structure parallel to the 

 schistose structure of the stratified rocks. 



(i) The intrusion of the porphyry took place at a period so distant 

 as to allow of imniense denudation. We in fact see at the surface 

 what was brought into its present condition at great depth. 



2. As to the Ore-deposits : — 



(J) The opening of the main fissures took place along the lines of 

 contact of dissimilar rocks, i. e. along lines of least strength. 



(Ic) The process of filling-in these fissures was, as regards the 

 pyrites, more chemical than mechanical ; but still not entirely 

 chemical. The great width of the fissures in certain places is here, 

 as elsewhere, more apparent than real. On the one hand there seems 

 to have been a great concentration of pyritous matter in solution, 

 probably derived from pre-existing pyritous schists, into the more 

 open parts of the fissures ; and on the other a gradual removal, 

 probably by the very same solutions, of much of the earthy matter of 

 the enclosing slates, so as gradually to produce a large mass of very 

 pure ore. The fact that schistose structure is often visible in the 

 interior of great masses of the pyrites appears to me to be in favour 

 of this theory of their mode of formation. The force of crystallization 

 itself, too, may have enlarged the cavities continuously by pressure 

 against the softened wall. 



(I) That the solutions circulating within the fissures had solvent 

 powers is evidenced by the softening and kaolinizatiou of the rocks 

 in the neighbourhood of the pyritous masses. Silica and lime appear 

 to have been remarkably absent from these solutions ; but in the later 

 vein-fiUings the former has been deposited to a small extent, and 

 sometimes accompanied by sulphate of baryta. 



{m) That the fissui'es were seldom really open to any great extent 

 is, I think, indicated by the rarity of distinct and well- formed crystals 

 of pyrites. At the same time the existence of fault-rock in the 

 immediate neighbourhood proves that open fissures must have existed 

 at one time. 



