15 



Microlites often show a tendency to build up compound forms possessing 

 more or less regularity. This is well seen in the Arran pitchstone, (1) where 

 minute needle-like bodies (belonites) group themselves so as to produce most 

 beautiful arborescent aggregates, reminding one of the forms assumed by 

 water as it crystallises on a window frame. (Fig. 6.) 



Fig. 6. (After Vogelsang.) 



Feathery forms and skeleton- crystals are also produced by the aggregation 

 of microlites. (Fig. 7.) 



Fig. 7, Skeleton Felspars in the ground-mass of the Cleveland Dyke, Preston. 



170 diameters. 



Magnified 



Another mode of aggregation is that known as the spherulitic. True 

 spherulites consist of crystalline fibres which radiate from some central point 

 and extinguish light under crossed nicols when their long axes lie parallel 

 with the vibration planes of either the polariser or analyser. They therefore 

 give rise to a black cross which remains stationary as the stage is rotated. 



It sometimes happens that spherulitic aggregates may be formed by the 

 inter-crystallisation of two minerals ; such, for instance, as quartz and felspar. 

 In these cases, the axes of elasticity in the different elements of the spherulite 

 may have different positions in relation to the spherulite as a whole, and the 

 structure may give rise to more or fewer arms than the number characteristic 

 of true spherulites, and their arms may not lie parallel with the vibration planes 

 of the nicols. Such structures Professor Rosenbusch (2) proposes to call 



(1) S. ALLPOET. On the microscopic structure of the Pitchstones of Arran. G.M., 1872. p. 1. 



(2) Zusammensetzung und Structur granitischer Gesteine. Z.D.G.G. 1876, p. 369. 



