STRUCTURES OF CRYSTALS 



1073 



equivalent to that of the flow, and where such streams encounter 

 large crystals they sweep round them in graceful curves : this 

 appearance in a rock is known as fluxion-structure. 



In certain glassy rocks nn'crolites are collected into more or 

 less spherical masses, exhibiting a radial structure, called spheru- 

 lites ; commonly these are not bigger than a pea, but sometimes 

 they are one or two inches in diameter ; they are then less regular 

 in shape and structure and are often named for distinction pyro- 

 merides. Chemical analysis often shows that they differ slightly 

 in composition from the base. Crystalline rocks also sometimes 

 exhibit a similar structure, e.g. the orbicular diorite of Corsica. 

 A spherulitic structure can be produced in a compact rock by subse- 

 quent heating, short of melting, and many glassy rocks in lapse of 

 time become ' devitrified ' by setting up an obscure confused crys- 

 talline structure. 1 



Masses of molten material mav, however, consolidate at a con- 



/ ' 



siderable depth beneath the surface of the earth ; in such cases the 

 distinction bet ween the first and second period > 

 of crystallisation is not generally so well 

 marked . 



A crystal is, in one respect, like an 

 organism it is affected by its environment. 

 The crystal modifies its surroundings, and 

 is in turn modified by them ; there is action 

 and reaction between it and its environment. 

 This remarkable property of all crystalline 

 bodies is well shown by the microscope. 

 Crystals are constantly found built up of 

 different layers or zones of material slightly 

 unlike in their optical characters, and thus 

 dissimilar in chemical constitution. This is 



the so-called zonal structure, and is common in the felspars and 

 augites in short, in nearly all minerals which admit of isomor- 

 phic replacement in their constituents (fig. 806). Its presence in the 

 case of the augites is often indicated by a difference in colour. This 

 structure may be experimentally produced by placing an artificial 

 crystal in a solution of a substance isomorphic with that of the 

 crystal. 



The microscope has rendered another great service, inasmuch as 

 it has enabled the petrologist to draw conclusions as to the physical 

 condition of the fused mass or magma at the time crystallisation 

 commenced. All chemists are aware that when crystals are deposited 

 from solutions at ordinary temperatures they usually contain small 

 cavities full of the mother -liquor. Now, the growth of crystals in 

 igneous rocks is exactly analogous to that in a supersaturated saline 

 solution. Portions of the fused mass become entangled, which on 

 cooling remain in a glassy condition, or ' become stony, so as to 

 produce what may lie called glass- or stone-cavities.' 2 When formed 



1 This subject is discussed in Quart. Joan/, (irol. Soc. 1885 (Presidential 

 address). 



2 Sorby, Quart. Journ. Geol Soc. 1858, p. 24-2. 



Fm. 800. Augite showing 

 zonal structure. (After 

 Zirkel.) 



