10 F. L. Stilhvell: 



For further help we must therefore depend on the less convincing 

 petrological evidences. Kickard (5) looked- upon the nionchiquites as 

 part of the newer basalts. The monchiquites, however, are quite 

 different from the newer basalt type, and are even more basic than the 

 older basalt series. 



The nearest petroo-raphical allies are found in some limited ultra-basic 

 lava flows in the Macedon district, e.i;'., King's (^)uarry type. Professor 

 Skeats and Mr. Summers (11) have placed these flows near the top of 

 Macedon series of tertiary igneous rocks. They have suggested the 

 existence of a sub-alkaline magma beneath the whnle of Victoria in 

 tertiary times, and progressive differentiation from it has produced 

 first the alkaline rocks and later the basic rocks. If this is correct and 

 the King's Quarry type is a differentiation product from this magma, 

 then the monchiquite dykes may well be its dift'erentiation products 

 also, because the monchiquite itself is a sub-alkaline rock. This 

 analogy suggests that the monchiquites are of mid-kainozoic age. 



Monchiquites are known to occur in a similar manner in other parts 

 of Victoria. 



Correlative evidence may therefore be produced in the future, but 

 till then the balance of evidence, such as it is, leans towards the con- 

 clusion that these dykes were intruded in mid-kainozoic times. 



Relation of the " Lavas" to the Gold Distribution. 



Only a study over an extended period of time can throw much light 

 on this question. Concrete cases, nowadays, of lavas in contact with 

 rich gold-bearing stone are rare, and many such instances must be 

 found and ejcamined before the problem can be thoroughly discussed. 

 Instances were probably not so rare during the earlier history of the 

 field when all the development work was in the shallower levels, and 

 it is more often the miner of the old days who asserts that the lava 

 exerts an influence on the gold. Tliere can be no doubt that such 

 belief would be assisted by the presence of the lavas in centre country, 

 and the lavas have been followed as guides to the unrecognised centre 

 country. Such is obvious in mines like the Pearl, where drives in each 

 successive level have been started on the lava. 



My contribution to this question is the record of observations in the 

 Ironbark Mine, Long Gully. Here, in sinking a winze in the centre 

 country of the Sheepshead line from the end of a crosscut about 700 

 feet long at the 4:80-foot level, a lava was found to split into two 

 branches, each about ten inches wide. Tlie branches opened to about 

 four feet, and tlien continued more or less parallel. Tlie lavas are the 

 dark, dense characteristic monchiquite, with streaks and segregations 

 of olivine altered to serpentine. The enclosed space was tilled with 

 quartz. The winze was down twenty-seven feet at the time of my first 



