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DECOMPOSITION OF BASALTIC ROCKS. 515 
-depth of an inch and a half. The colours, beginning within, are dirt- 
brown, grayish-yellow, ochre-yellow, brownish-red ; and they are evi- 
dently dependent mostly on changes in the condition of the iron 
which the rock or its minerals contain. 
When the rock includes much chrysolite, the results of decomposi- 
tion in some instances give a fissile or micaceous appearance to the 
rock. At Prospect Hill, five miles west of Paramatta, this change is 
in progress. The rock is a black ferruginous basalt of homogeneous 
aspect, breaking with a smooth fracture and no appearance of crystal- 
lization. It contains chrysolite; but the grains are small and not 
apparent except on very close examination. ‘The decomposed basalt 
overlies the columnar basalt, as represented in the figure here given, 
which is a section from the quarry. 
Were we unable to trace the transi- 
tions, and distinguish the columnar 
structure through the whole, we should 
scarcely suspect its basaltic origin. In- 
deed it was pointed out to me as an in- 
stance of mica slate overlying basalt. 
Particles of rusted mica, as they seemed, 
were distinct, and it much resembled a | 
decomposing variety of that rock. On 
close inspection and an examination of 
the rock in different stages of change, it became evident that the 
pseudo-mica was nothing but altered chrysolite, which had rusted 
from partial decomposition, and split into thin cleavage scales. 
The crystals of chrysolite have evidently a parallel position in the 
rock, and hence the plane of easiest cleavage lies in the same direc- 
tion, or, as the cleavage shows, parallel with the upper surface, that 
is, at right angles with the vertical axis of the columns. The passage 
from the compact to the decomposed rock is, in this case, unusually 
abrupt. Alteration takes place, (through the elimination of oxyd of 
iron as before suggested,) slowly at the surface, which, therefore, 
chips off as soon as decomposed, and exposes a new portion. This 
sudden transition may, in part, proceed from the absence of any 
natural planes of fracture, (which are brought out when there is a 
concentric structure,) and perhaps in part also from the presence of 
chrysolite. The layer of pseudo-mica schist is in some places five feet 
thick, and has a rusty brownish colour. Above, it passes into three 
