MR. J. H. COLLINS OX THE SUDBURY COPPER-DEPOSITS. 837 



posits themselves, are so different in age and in mineral character, 

 yet the modes of origin have apparently been very similar. In 

 each region we have highly inclined stratified beds penetrated by 

 dykes of igneous origin which have followed the stratification so 

 closely that they present the appearance of interbedding, and 

 suggest a contemporaneous origin until very closely examined. The 

 following conclusions seem to me to be fully warranted in both 

 regions : — 



(1) The rocks immediately enclosing the ore-deposits were 

 originally, or at a very early period, pyritous and probably cupri- 

 ferous. 



(2) The intrusions of igneous matter gave rise to lines of weak- 

 ness along the planes of contact. 



(3) Subsequent Assuring and, to a certain extent, faulting oc- 

 curred at these contact-planes. 



(4) The fiUing-in of these fissures was mainly by solution from 

 the pyritous and cupreous material of the enclosing stratified rocks. 



(5) There is, in places, a pyritous breccia indicating a partial 

 mechanical filling. 



(6) There is, in places, a concentration of mineral matter in 

 those portions of the '^ country rock" which adjoin the more solid 

 deposits occupying the fissures. 



(7) The formation of rich veinlets or '• leaders " of ore within 

 the masses has been the result of subsequent operations, probably 

 at many very different times. These veins appear to occupy minor 

 faults and shrinkage-cracks, and to have been filled by segregation 

 of more richly cupreous material derived from the main masses of 

 pyrites. 



(8) Abundant evidence of partial movements within the masses 

 of pyrites is afi'orded by the numerous slickensides which are every- 

 where and continually met with. 



So far, the phenomena observable in the two sets of deposits are 

 parallel, if not absolutely identical. The following differences may 

 now be noted : — At Sudbury the stratified rocks are Huronian, the 

 intrusive masses dioritic, and the mineral deposits mainly pyrrho- 

 tite, a monosulphide of iron, or nearly so, with less than 40 per cent, 

 of sulphur when free from foreign matter. At Rio Tinto and in 

 the south of Spain generally the stratified rocks in which the 

 pyritous deposits occur are IJpper Devonian, the intrusive masses 

 are generally, if not always*, quartz-porphyries, and the deposits 

 always consist in the main of bisulphide of iron with 50 per cent, 

 or more of sulphur, pyrrhotite being unknown. In addition to the 

 great differences of age and of mineral composition, we may ob- 

 serve in these two series of deposits the following differences iu 

 their surroundings : — 



a. At Sudbury there is little or no evidence of kaolinization of 

 the felspathic ingredients of the country rocks in the immediate 



* According to E, Wimmer some of the deposits in the neighbourhood of 

 Tharsis are associated with dioritic intrusions, 



Q. J. G. S. No. 176. 3 k 



