MINERALS IN BASALT. ilk 
coaly rock are entangled in the basalt, and small hand specimens may 
be broken off, showing the two combined. 
Although the heat has been sufficient to alter the rock for one hun- 
dred and fifty yards from the dike, no approximation to granitic rocks 
was anywhere to be found. The chert had no trace of a crystalline 
character; neither had the sandstone any peculiar structure beyond 
what is understood from the expression baked sandstone. Its opaque 
clayey particles were similar to those in the unchanged rock, except 
that they were whiter and more like particles of pipe-clay. 
Owing to the small extent of the island, we cannot trace the effects 
of heat beyond one hundred and fifty yards from the dike. The main 
shore to the southward is about three hundred yards distant, and here 
the clays and coal were unaltered. The transfusion of the heat to such 
a distance from its source could only take place under water, for we 
well know how small a thickness of dry firebrick is needed to confine 
the heat of the hottest furnace. The waters of the ocean would be heated 
by the erupted rocks, and the moisture of the clayey and sandstone 
layers, and of included cavities among them, would aid in receiving 
and transmitting the heat. So that a clay which, if dry, would have 
not been affected to a distance of a foot, might thus have been baked 
to a distance of some hundred feet. 
Minerals in Basalt.—Besides feldspar, augite, and chrysolite, these 
rocks contain geodes of quartz and chalcedony, calcareous spar, stil- 
bite, and chabazite. 
Large geodes of quartz and chalcedony occur near Kiama, and at 
Rocky Cove, three miles to the southward ; and oc- 
casionally the quartz is amethystine. ‘The geodes \ 
in these regions lie mostly in an east-and-west or 0 : 
east-southeast and west-northwest direction. ‘They ) 
have mostly an ear-drop form, as in the annexed ) 
figure, and have the smaller end pointed to the east- 
ward. ) ) 0 
The calcareous spar of the Kiama region occurs at 
times in large crystallized masses, two feet in dia- 
meter, occupying cavities in the basaltic conglomerate, and there are 
also small crystals in some cavities. Stilbite is met with in the same 
rocks ; though the specimens are not of much interest to the mineralo- 
gist. Chabazite occurs in the hills near Puenbuen in small unmodi- 
fied rhombohedrons. 
Decomposition of Basaltic Rocks.—The basaltic rocks of Australia 
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