12 



(2) Crystallites. 



(3) Mierolites, microlitic aggregates, skeleton crystals, crystalline grains, 



crystalline masses of irregular form, crystals, and crystal fragments. 



GTLASS. A glass is a homogeneous solid whose external form and internal 

 structure are to a very great extent independent of its chemical composition. 

 A crystal, on the other hand, is a homogeneous solid whose external form and 

 internal structure are related to its chemical composition ; to the circum- 

 stances under which it has been developed; and in some cases also to the 

 changes which have taken place in its environment since it was produced. 

 The composition of a glass cannot as a rule be expressed by a definite chemical 

 formula. A crystal is of necessity a definite chemical compound. Glass 

 possesses no double refraction, except when subjected to strain. (1) It therefore 

 appears uniformly dark between crossed nicols, as the stage is rotated under 

 parallel rays of polarized light, and gives no interference figure with 

 convergent polarised light. Crystals, on the other hand, unless they belong 

 to the cubic system, invariably possess double refraction in certain directions, 

 and, in those sections which appear uniformly dark between crossed nicols 

 with parallel rays of polarised light, (2) give the interference figures of uniaxial 

 or biaxial crystals in convergent polarised light. 



It thus appears that the glassy and crystalline conditions stand, in a 

 certain sense, in opposition to each other. Experiment and observation 

 indicate that the conditions requisite for the formation of a perfect glass are 

 those dependent on the rapid cooling of a perfectly fused mass of rock. 

 Under these conditions definite chemical compounds are unable to individualise 

 themselves by crystallisation. The element of time is involved in crystal- 

 building, and here that element is wanting. 



The crystalline constituents of igneous rocks may be made to assume the 

 glassy condition by fusion and rapid cooling. When this is done the specific 

 gravity of the glass is always found to be less than that of the crystal from 

 which it is produced. The following cases serve to illustrate this point. (3) 



doubly-refracting particles and a glassy or micro-felsitic base by the use of thinner sections and 

 higher powers. 



(1) F. RCTTLEY. On strain in connection with crystallisation and the development of perlitic 

 structure. Q.J.G.S.. Vol. XL., p. 340. 



T. WALLER. The phenomena of strains observable in obsidian, G.M., 1885, p. 91. 



(2) A section of a biaxial crystal cut at right angles to one of the optic axes, when examined 

 with an ordinary petrographical microscope, does not remain dark in all positions as the stage is 

 rotated. This is owing to the fact that the rays of light are not strictly parallel. 



(3) Compiled from Roth's allgemeine und chemische Geologie. Vol. II., p. 52. Berlin, 1883. 

 The student is referred to this work for fuller detail and for references to original papers. 



