MELLO: mCROSCOPICAL STRUCTURE OF ROCKS. 
159 
enable us to determiue which elements of the rock cooled down first, 
and will also show the direction of motion in its substance. Micro- 
lites, as these microscopic crystals are called, are extremely common 
in most of the basic rocks, as in dolerite, basalt, obsidian, and others, 
microlites of augite being veiy fi'pquent. We also meet with micro- 
lites of kematite in felspar, magnetite too is continually present. 
Amongst other minute forms found in the igneous rocks are some 
with no definite crystalline structure, some of those which are termed 
crystallites are semi-crystals, or, as Mr.Teallsays, "bodies intermediate 
between glass and true crystal." Then again we shall in some cases 
detect what is known as sphenilitic structure, crystalline fibres radiat- 
ing from a central point and which give rise to a dark interference 
cross which remains stationary as we rotate the object. Spherulitic 
structure is often found in the glassy rocks, and is a mark of devi- 
trification, the gradual alteration of a glass into a more or less crys- 
talline condition, and the existence of spherulites tend to show that 
the rock in which they are developed has at one period been glassy. 
Another point to be observed is that although the typical 
character of a crystal is homogeneity, we more often find in the case 
of those crystals which occur in rock sections that perfectly trans- 
parent homogeneous crj'stals are the exception not the rule, and that 
in most cases the crystals under observation are full of foreign 
material; inclusions, some of which may have been contemporary 
with its formation, but others which have been subsequently produced, 
some of these will often afford valuable indications as to the con- 
ditions under wliich the crystal was formed, and the changes which 
have since its formation passed over it. 
It is sometimes diflicult to tell whether a particular crystalline 
body is a single crystal or merely a crystalline aggregate. The polari- 
scope will at once reveal the ditference by breaking up the aggregate 
into its component parts, which will appear variously coloured ; a 
single crystal being of an uniform or nearly uniform tint, its difierence, 
if any, depending merely on its unequal thickness. Some observations 
may be made vdth. the polarizer alone, the analyzer being removed, thus 
the dichroism of certain minerals is determined in this way. Horn- 
blende may by this means be discriminated from augite, biolite is 
