88 ON FELSITES AND ASSOCIATED ROCKS. 



of the schist \vithout dislocation. These lenticuhir masses 

 have been looked ui)on as segregations of mineral along 

 crevices or lines of weakness in the rock. It is rather 

 difficult to imagine cavities of the required size existing 

 in the rock in readiness for filling up with mineral ; but 

 there is nothing improbable in supposing the parting planes 

 of the schists to be the first channels for the precipitation 

 from solutions of their metallic contents. The process 

 of replacement might then very well start from these 

 channels and remove the country rock on either side, 

 leaving ore in its place. The suggestion that the lenticular 

 ore bodies represent old lake bottoms has not the testimony 

 to support it which can be adduced in favor of the metaso- 

 matic hypothesis. The sulphides of the Rio Tinto Mine 

 in Spain which have been appealed to are mostly bodies 

 filling fissures which separate slates and intrusive quartz 

 porphyry ; and it is only iron ore lying in horizontal beds 

 of miocene age with plant remains which can be referred 

 to a sedimentary origin. Those conditions are not com- 

 parable with the mineral zones on Mounts Black and Read. 

 Here the ore bodies follow the dips. and foliation planes 

 of the enclosing schists which on Mount Read dip easterly 

 at a high angle with a strike from 10° to 20° W. of N. The 

 lenticular forms of these bodies are suggestive of replace- 

 ment having gone on pari jjas.sii with the operation of a 

 solvent. They differ from true lodes in always being 

 found conformable to the surrounding schist, and from 

 both fissure and segi'egation veins in having no gangue or- 

 matrix different from the country rock. The foliation 

 planes have apparently served as initial channels for the 

 mineralised solutions which attacked and removed the 

 schist on each side, and left their mineral contents in situ,- 

 It is obvious that in such a process there w^ould be a 

 beginning : the attacking solution would attain a maximum 

 of power and gradually decrease to a minimum. The result 

 would be a lens-shaped body of mineral. The question 

 presents itself, are such deposits as permanent as true 

 lodes ? But this is hardly the appropriate form for such a 

 query. It w^ould be more proper to ask, are these ore 

 deposits as reliable as the pitches or shoots of ore in true 

 lodes ? We know very well that, though mineral veins or- 

 lodes go dow^n to apparently quite inaccessible depths in 

 the earth's crust, the courses of ore which they contain are 

 inconstant and irregular. A metalliferous zone is followed 

 by a barren one, or vice versa. Therefore, to institute a 

 just comparison, we must imagine the partings of the 



