WITH THE MICROSCOPE. 177 



form which may be produced in particular circumstances. Such 

 facts may show for example that some round bodies, like oolitic 

 grains, have been formed by the external growth of crystals radiating 

 from a central nucleus, whilst others, like those so common in 

 meteorites, were formed in an entirely different manner, and have the 

 structure of round bodies which crystallised afterwards. * 



2G7. Crystals of one Mineral enclosed in another. The enclosure 

 of crystals of one mineral inside another presents us with a long series 

 of interesting facts, described in a work specially devoted to that 

 subject.t Any one who has not examined the microscopical struc- 

 ture of some rocks, would hardly believe the extent to which this 

 occurs. The minerals in erupted lavas are often full of minute 

 crystals, and it is easy to understand why chemical analyses should 

 frequently give such anomalous results. Care must sometimes be 

 taken not to confound such included minerals with cavities, and by 

 using polarised light we generally distinguish them, though it must 

 be admitted that transparent crystals having no double refrac- 

 tion might look very much like cavities filled with some liquid. 

 The most satisfactory proof of their being cavities is the formation of 

 a bubble when the temperature is reduced, but in other cases 

 we may observe whether the form is that of an independent minute 

 crystal, or related to the shape of the larger crystal, as is the case 

 with cavities. 



268. of the Cavities in Crystals. -The study of these cavities 

 constitutes one of the most important branches of our subject, since 

 by means of them we may often learn under what conditions the 

 rock was formed, as I have shown in a paper published some years 

 ago.| I think that I cannot do better than give the short abstract 

 printed at the time. 



" In this paper the author showed, that, when artificial crystals are 

 examined with the microscope, it is seen that they have often caught 

 up and enclosed within their solid substance, portions of the material 

 surrounding them at the time when they were being formed. Thus, 

 if they are produced by sublimation, small portions of air or 

 vapour are caught up, so as to form apparently empty cavities ; 

 or, if they are deposited from solution in water, small quantities of 

 water are enclosed, so as to form fluid-cavities. In a similar manner, 

 if crystals are formed from a state of igneous fusion, crystallising out 

 from a fused-stone solvent, portions of this fused stone become 

 entangled, which, on cooling, remain in a glassy condition, or 



* On measuring the angles of crystals, see part III. 



f Sochting. Einschliisse von Mineralen. Freiberg, 1860. 



% Quart. Jour. Geol. Soc., vol. XIV, p. 453. 



N 



