150 Notes on the Area of Intrusive Rocks at Dargo. 



indicate probably fissuring tbrough homogeneous rocks, or 

 without shifting of the sides of the fissure. As I have 

 before said, the lodes which fill these fissures do not, as a 

 rule, extend any distance down into the intrusive rock 

 masses, but thin out, and are lost. The fissures were clearly 

 formed at that time when the sedimentary crust was raised, 

 and its strata opened and faulted during the time of the 

 plutonic activity to which I have so frequently had occasion 

 to refer in this series of papers. This took place pro- 

 bably at the close of the Silurian age; but it does not follow 

 that these fissures were then filled with the quartz lodes 

 and the minerals which we now find in them; nor can it be 

 assumed that the fissures were opened once only; on the 

 contrary, 1 think that, as to the veins at the Exhibition 

 mine, they have been probably opened and widened by a 

 second addition of quartz. I have observed places where 

 the quartz was divided by seams carrying ores parallel to 

 the walls of the lode, and a sample which I sliced and 

 examined under the microscope confirmed this belief. I 

 found the quartz to be crystallised, and that one growth of 

 crystals started from the walls, being filled in by a confused 

 mass of imperfect crystals in the centre. It must, however, be 

 remembered in connection with this subject that fissuring 

 of the rocks forming the contact zone would probably follow 

 any of the great changes to which the plutonic masses, 

 together with the adjoining sediments would be subjected, 

 through cooling down of the former or general subsidence 

 of the crust. The periods of time during which all the 

 changes took place, from the invasion of the sediments until 

 the cessation of plutonic activity in tha.t area, were evidently 

 geological periods, and not to be measured in years. 



The fissures at the Exhibition mine are narrow, and the 

 lodes do not include, as is tlie case elsewhere, fragments of 

 the bounding rocks which have fallen in during the move- 

 ments of the rocks, and thus become highly mineralised 

 during the lode formation. 



The gold in these contact lodes is almost invariably asso- 

 ciated with large amounts of ores, such as arsenical and 

 ordinary pyrites, copper pyrites, galena, and, more rarely, 

 blende. Near the surface these ores become decomposed, 

 and the honeycombed quartz which remains retains the 

 gold, which was formerly included in the sulphides and 

 arsenides in its cavities, or embedded in hydrated ores. 

 The greater part of the ores and the gold are found within 



