58 Personal Observations 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, asin 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 érzpoli or steinmark, a finely 
‘pulverised earthy deposit, of a greyish yellow colour, and a dry 
tigeh 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 lamine 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 endeayoured 
“to describe, are not so marked, or so fully developed as the 
latter. 
"Between 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 
nn, the country exhibits every indication of being of an 
~ guriferous character. 
