58 



Personal Ohservations in the 



second eruption took place^ resulting in the production of the 

 peculiar columns mentioned in the preceding paragraph. The 

 origin of their formation reminded me of the phenomenon 

 attendant on the refining of silver, as in both cases a discharge 

 of gases must have taken place. The columns are solid, 

 although they contain a considerable number of hollow 

 concretions (septarians), from one to three feet in diameter, 

 which are, however, filled up with ^rzpoK or steinmark, a finely 

 pulverised earthy deposit, of a greyish yellow colour, and a dry 

 harsh texture. This tripoli, whose component parts are as 

 follows: — silica, 80 ; alumina, 2; iron, 8 ; water 5 ; sulphurous 

 acid, 5 — ^is, I think, mostly derived from the decomposition of 

 silica ; It Is very soft, and Is much valued in other parts of the 

 world, as a polishing material for hard metals and precious 

 stones. 



Many of the septarians are fractured or burst, and their 

 internal structure thus exposed to view. I attribute these 

 fractures to have originated from the following cause, viz., 

 from the decomposition of the outer surface of the columns 

 by the usual atmospheric action. The last laminte being at 

 length penetrated, the rains gained access through the fissures, 

 and the expansive power of the tripoli, arising from the 

 moisture, ultimately burst the septarians ; and their contents, 

 issuing from the opening, on to the rocks beneath, dotted 

 them over with white spots, some of which are still obser- 

 vable, though the greater part of them are wholly erased by 

 time. The soil between the columns, which are so numerous 

 and thickly disposed as scarcely to allow a rider to pass 

 between, arises from the decomposition of the dolerite, and 

 is extremely rich. The physical conformation of Mount 

 Macedon is identical with that of Diogenes' Mount, although 

 the peculiarities of the former, which I have here endeavoured 

 to describe, are not so marked, or so fully developed as the 

 latter. 



BetAveen Dr. Baynton's and Mr. Perry's run, and the Mie 

 Mie Inn, are extensive basaltic plains. Between Mr. Pohlman's 

 and Mr. Perry's I found boulders of magnesite, about one 

 foot in diameter, and similar in appearance to the stone 

 which I had previously observed on Brock's run. 



The steep banks of the Campaspe consist partly of basalt 

 and of slate, with quartz scattered over the surface. ^ East- 

 ward of the Campaspe, from Dr. Baynton's to the_ Mie Mie 

 Inn, the country exhibits every indication of being of an 

 auriferous character. 



