ON THE MICROSCOPICAL STRUCTURE OF ROCKS. 
13 
Augite . — Augite is a mineral as important in rock formation 
as hornblende, and is as constantly present in dolerites as the 
latter is in diorites. In the microscope it usually is very 
recognisable, on account of its brownish or brownish yellow 
tinge, and its crystalline form is frequently very distinct, and 
well defined from the surrounding mineral matter. Some of 
the crystals will he perfect at one extremity and broken at the 
other. The crystals are also often grouped in masses, or a 
number of small crystals will be united so as to form apparently 
a single one ; in these cases polarised light will show the com- 
posite nature of the seemingly simple crystal. Dislocated 
crystals of augite are very frequent in some rocks. Augite 
crystals have a somewhat broken-looking surface, and irregular 
flaws may be seen in them with transmitted light. Vast quan- 
tities of minute well-formed transparent light yellowish brown 
crystals of augite sometimes abound in basalt rock, also needles 
of this mineral, which occasionally are curiously shaped, being 
curved at one end like a hook, or swollen like a club, or some- 
times both extremities will be split into dichotomous points. 
Very numerous microlites are enclosed in augite crystals ; 
amongst them are leucite, biotite, magnetite, &c. Sometimes 
the augite crystal will be a mere thin wall, enclosing a multi- 
tude of microscopic minerals. Cavities of all sorts are also 
frequent in augite. The double refraction of augite is powerful, 
and in thin sections it polarises with brilliant colours. 
Diallage . — Diallage, when crystallised, corresponds with 
augite, but more frequently it is found in granular aggrega- 
tions, and fills up or borders cavities in the matrix. Its usual 
colour is green, but it is sometimes brownish, on account of 
enclosed mineral matter. Sections of diallage show a distinct 
striping parallel to the chief axis of the crystal, and a concen- 
tric, shaley structure is sometimes denoted by little colour 
rings. Between its laminae a secondary formation of calcite is 
occasionally met with, also hornblende, but this is an original 
formation. It encloses, like augite, various other minerals, 
and on its surface it often exhibits a kind of metallic lustre. 
Its slight absorptive optical properties easily prevent its being 
mistaken for hornblende, with which its laminated texture 
might possibly lead some to confound its sections. 
Melilite . — Melilite is a mineral sometimes found coating 
porous cavities in lavas, and also as a chief component of some 
of the basic rocks, but is never met with in the felspathic 
basalts. Its appearance in thin sections will be in quadran- 
gular or long rectangular forms, but somewhat ill-defined ; more 
often it is met with in irregular aggregations. Melilite has a 
coarsish surface striping, always in the direction of the chief 
axis — that this is only a surface striping is proved by its not 
