58 



CRYSTALLOGRAPHY. 



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 species of 

 quartz in twins made up of the forms li 

 and — li (or —1). In fig. 10 the shadtd 

 parts of the pyramidal planes are of th8 

 form — 1, and the non-shaded parts of 

 JZ. Each of the faces is made up partly 

 of M 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 crys- 

 tals of quartz, the striations of planes i 

 are owing to oscillations between pyram- 

 idal and prismatic planes while the for 

 mation of the latter was in progress. 



3. CRYSTALLINE AGGREGATES. 



The crystalline aggregates here included are the simple, not 

 the mixed ; that is, they are those consisting of crystalline in- 

 dividual? of a single species. 



The crystalline individuals may be (1) distinct crystals ; (2) 

 fibres or columns ; (3) scales or lamellae ; or (4) grains, either 

 cleavable or not so. 



1. Consisting of distinct crystals. — The distinct crystal may 

 be either long or short prismatic, stout or slender to aticidar 

 (needle-like), and capillary (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 surface!:'. ; 

 (d) about centres. The various kinds of aggregates thus made 

 are : 



a. Filiform. — Thread-like lines of crystals, the crystals ofteD 

 not well defined. 



b. Dendritic. — Arborescent 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, etc. 



Fig. 11 represents, much magnified an arborescent form of 

 magnetite occurring in mica at Pennsbury, in Pennsylvania. 

 A rborescent delineations over surfaces of rock are usually called 



