MELLO: MICROSCOPICAL STRUCTURE OF ROCKS. 
169 
Aragonite, another form of calcic carbonate, differs altogether 
from calcite by its cleavage, which, instead of being rhomboidal, is 
conchoidal, and under the microscope lias a foliated appearance, 
which is well seen in some sections of Carrara marl^le. 
Another secondary formation occasionally found filling amyg- 
daloidal spaces is chalcedony, a variety of quartz. It is an exquisitely 
beautiful object when polarised, showing a minute crystalline radiated 
structure, gorgeously coloured, and presenting also fine illustrations 
of interference spectra. 
Delessite is a mineral wJiicli is not unfrequently found in amyg- 
daloids, either completly filling up cavities or else coating their sides. 
Under the microscope it has a concentric banded structure, with a 
radiating growth between the bands, or rather cutting through them. 
The concentric lines follow tlie outline of the cavities. The colour of 
this mineral is green, and it is transparent and pleochroic. Between 
crossed prisms tlie radial forms of delessite, especially sections across 
amygdaloidal enclosures, show the interference cross very beautifully. 
Amongst the secondary formations in the volcanic rocks zeolites 
of various species are common, both filling microscopical as well as 
macroscopical cavities. In all of them we may observe the same 
tendency to a concentric or fibrous structure. Their natural colours 
are varied, yellow, brown and green tints being the most common, as 
well as white. 
In studying the microscopical structure of igneous rocks, certain 
facts may be deduced, amongst which may be mentioned, that it will 
be possible to gather whether the rock was rapidl}^ or slowly cooled ; 
a rock full of minute crystals will have consolidated more rapidly 
than one in which these are large and well-formed. And rocks 
abounding in microlithic crystals may be considered to have solidi- 
fied near the surface, or to have been ejected as molten lavas; whereas 
those in Avhich the crystals are large would be slowly formed, cooling 
very gradually under pressure and probably at a considerable depth 
below the surface. Then, again, we shall gather that those crystals 
which are most perfectly developed will have been the first formed 
ones, and these will often be found to have considerably modified the 
formation of the others in the same rocks. 
