ON TIIE MICKOSCOPICAL STRUCTURE OF ROCKS. 
17 
purely aqueous deposits. In its microscopic forms it shows a 
banded and delicately iridescent structure, the bands coinciding 
with the rhomboidal shape of its crystals. This structure is 
beautifully displayed in many marbles. Calcite often is found 
filling the amygdaloid cavities of trappean rocks, and is fre- 
quently surrounded in them by a green chloritic coating. 
Water cavities are abundant in it, proving it to be a secondary 
product deposited from watery solution in the empty spaces 
which were originally gas bubbles in these rocks. 
Aragonite . — Aragonite, another form of calcic carbonate, 
differs altogether from calcite by its cleavage, which, instead of 
being rhomboidal, is conchoidal. Under the microscope it has 
a foliated appearance. In some sections of Carrara marble this 
is well shown, and if this is the general form of that marble, it 
may serve to explain its admirable adaptability to the purposes 
of the sculptor. 
Chalcedony . — Chalcedony is occasionally found in amygdaloid 
spaces, as well as forming the mass of agates, and is an ex- 
quisitely beautiful object in the polariscope, showing a radiated 
structure which displays the most gorgeous colours, as well as 
fine illustrations of interference spectra. 
Delessite . — Delessite is another mineral not unfrequently 
found in amygdaloids, either completely filling up the cavities 
or else coating their sides. It has a concentric banded struc- 
ture, with a radiate growth between the bands, or rather cutting 
through them. The concentric lines follow the form of the 
cavity in which the delessite is found. This structure is 
probably owing to intermittences of formation, and may be 
compared with the season rings of trees. The beautiful mossy- 
looking growth in some Indian agates has been considered to be 
delessite. The colour of this mineral is green, and it is 
transparent and pleochroic. Between crossed prisms the radial 
forms of delessite, especially sections across amygdaloid forma- 
tions, show the interference cross very beautifully in the pola- 
riscope. 
Zeolites .- — There are other secondary formations found in 
igneous rocks besides those mentioned. Zeolites of various 
species are common. All these formations may be found in 
microscopical as well as in the larger amygdaloidal cavities. 
Amongst them all we may observe the same tendency to a con- 
centric or fibrous structure. Their natural colours are very 
various ; yellow, brown, and green tints prevail, together with 
white or colourless specimens, which may be calcite, aragonite, 
or zeolite, and sometimes chalybite. 
Aqueous or Sedimentary Rocks.— Before bringing this 
paper to a close, a few words may be added upon the general 
characters of the aqueous rocks under the microscope. We are 
VOL. XIV. — NO. LIV. C 
