13 



The same relation necessarily holds between the specific gravity of the 

 glass formed by the fusion of a holocrystalline rock and that of the rock itself. 



Specific Gravity Specific Gravity 



of Rock. of Glass. 



Granite 2-680 2-427 



Syenite 2-710 2-43 



Diorite 2-779 2-608 



Porphyrite 2-768 2-486 



Gabbro 2-898 2-641 



We may conclude from these facts that the devitrification of a glassy 

 rock is accompanied by an increase in density, and that, in the case of the 

 complete devitrification of a glass having the composition of the granite 

 in the above table, the increase would be about 11 p.c. (1) 



Since an increase in density accompanies devitrification, it may reason- 

 ably be inferred that pressure will tend to bring about devitrification. 



The most perfect illustration of a natural glass is furnished by f ulgurite, (2) 

 the material which results from the fusion of rocks and minerals by lightning. 



An eruptive mass of any composition may assume the glassy form under 

 suitable conditions of consolidation. Some rocks, such as obsidian and 

 tachylite, are almost entirely formed of glass ; others are composed of crystals, 

 crystalline grains and crystal fragments embedded in a glassy base. Crystallites, 

 microlites, microlitic aggregates and skeleton crystals are frequently present 

 in the glassy rocks, and in the glassy base of the semi-crystalline rocks. 



CRYSTALLITES are bodies formed in consequence of the attempts of some 

 definite chemical compound to crystallise under unfavourable circumstances. 

 They represent matter in a state which is intermediate between that of a glass 

 and that of a true crystal. They occur not only in natural rocks but also in various 

 artificial products; such as slags, blowpipe beads and the refuse material of 

 glass works. The conditions of their formation have been determined experi- 

 mentally by Vogelsang, a brief account of whose researches will now be given. 



After many trials sulphur was the substance finally selected by Vogelsang 

 for his experiments. This substance is readily soluble in bisulphide of carbon, 

 out of which it crystallises in the rhombic form. If the process of crystalli- 

 sation be followed under the microscope nothing definite can be made out as 

 to the initial stages of crystal-development. The first objects which appear are 

 definite crystals of sulphur, and these grow by accretion in the usual manner. 

 If, however, the solution of sulphur be thickened with Canada balsam, then, 

 provided the proper proportions of the different substances have been 

 employed, some very interesting phenomena may be observed as the 



(1) Mr. Allport has described some ancient devitrified pitchstones and perlites from Shropshire, 

 and, in his paper (Q J. Gr. S., Vol. XXXIII. p. 449), he gives an analysis of one of these rocks, 

 and quotes for comparison the analyses of two Hungarian glassy rocks. There are no essential 

 differences between the three analyses. On referring to the work from which the analyses of 

 the Hungarian rocks are quoted, it will'be found that their specific gravities are 2'428 and 2-384. 

 That of the devitrified Shropshire rock is 2 - 62. The difference between the specific gravities of 

 the devitrified and glassy rocks implies an increase of 9 p.c. during the process of devitrification. 



(2) A. WICHMANN. Ueber Fulgurite. Z.D.G.G., 1883, p. 849. 



J. S. DILLEB. Fulgurite from Mt. Thielson, Oregon. A.J.S., 1884, p. 252. 

 F. RUTLEY. On Fulgurite from Mt. Blanc. Q. J.G.S., vol. XLL, p. 152. 



