16H 
MELLO: MICROSOOPIGAL STRUCTURE OF ROCKS. 
dodecahedral forms which are often very transhicent, although they 
frequently occur filled with microlites and cavities, both gaseous, 
fluid, and glassy. 
Another mineral occasionally present in some of the 
igneous rocks is nosean or hauyne, of which lapis lazuli is the 
well known blue variety. Its crystals are generally quadrangular 
or hexagonal, often irregular, and very generally filled with minute 
enclosures, which often form regular concentric bands, symmetri- 
cally folloAving the planes of the crystal. 
]^]pidote is of very common occurrence in granite and 
other rocks. It is frequently seen in elongated crystals, clouded with 
a dirty green colour. In some specimens its structure is radiated and 
fibrous, and polarises gTeen, yellow and brown, in most cases. Now 
and then this mineral occurs in large crystals, filling up cavities in 
some rocks, as for instance in the diorite of Quenast, in Belgium. 
Iron is abundant in all the igneous rocks, occurring as magne- 
tite, luvmatite or specvdar iron, and occasionally as titaniferous 
iron. Curious little opaque clusters of magnetite are frequent, 
and translucent plates of haematite abound in some varieties of fel- 
spar: it is the light reflected from these that gives the variety of 
oligoclase known as sunstone, its beautiful opalescence. 
x\mongst alteration products, one of the most important is 
serpentine, which is often found in enormous masses. Under the 
microscope it has a very beautiful appearance, showing a confused 
reticulated not work of varied tints, and brush-like aggregations of 
needles may often bo observed on the outer edge pointing inwards. 
The microscopical behaviour of calcite has been previously 
noticed. This mineral is very frequently found as a secondary 
product, filling up cavities and fissures in the igneous as Avell as in 
other rocks, it is invariably a deposit from aqueous solutions. Often 
the amygdaloidal vesicles in trappean rocks, especially dolerite, are 
filled with calcite, and this is frequently surrounded by a coating of 
green chloritic material. AVater cavities abound in the calcite, 
proving its secondary origin in such rocks, from solution in water, 
which has ])assod into the empty sj)aces which originally were gas 
bubbles. 
