58 ; CRYSFALLOGRAPHY. 
In some twin crystals the two component parts of the crystal 
are not united by an even plane, but run 
into one another with great irregularity. 
Cases of this kind occur in the speciesof 
quartz in twins made up of the forms £ 
and —/? (or —1). In fig. 10 the shaded 
parts of the pyramidal planes are of the 
form —1, and the non-shaded parts of 
ft. ach of the faces is made up partly 
of & and partly of —1. The limits of 
the two are easily seen on holding the 
crystal up to the light, since the —1 
portion is less well polished than the 
other. In this crystal, as in other erys- 
tals of quartz, the striations of planes 4 
are owing to oscillations between pyram- 
idal and prismatic planes while the for- 
mation of the latter was in progress. 




















38. CRYSTALLINE AGGREGATES. 
The crystalline aggregates here included are the simple, not 
the mixed; that is, they are those consisting of crystalline in- 
dividuals af a single species. 
The crystalline individuals may be (1) \distine! crystals ; (2) 
fibres or columns ; (3) scales or lamellae; or (4) grains, either 
cleavable or not 50. 
1. Consisting of distinct crystals —The distinct crystal may 
be either long or short prismatic, stout or slender to acicular 
(needle-like), ‘and capulary (hair-like) ; or they may have any 
other forms of crystals. They may be aggregated (a) in lines; 
(b) promiscuously with open spaces; (c) over broad surfaces ; 
(¢d) about centres. The various kinds of aggregates thus made 
are: 
a. Liliform.—Thread-like lines of crystals, the crystals often 
not well defined. 
b. Dendritic.—Arboresceut slender spreading branches, some- 
what plant-like, made up of more or less distinct crystals, as in 
the frost on windows, and in arborescent forms of native cop- 
per, silver, gold, ete. 
Fig. 11 represents, much magnified an arborescent form of 
magnetite occurring in mica at Pennsbury, in Pennsylvania. 
Arborescent delineations over surfaces of rock are usually called 
