SORBY STRUCTURE OF CRYSTALS. 4/9 



slags, their very nature proves the igneous origin of the minerals 

 containing them. This is especially the case with glass-cavities ; for 

 nothing but igneous fusion could so liquefy the enclosed glass that 

 perfectly spherical bubbles could be produced. 



Besides stone- and glass-cavities, the minerals of erupted lavas con- 

 tain gas- or vapour-cavities, as if they had caught up small quantities 

 of gases and vapours that were in contact with them ; but I have 

 never found any fluid-cavities, and hence the purely igneous origin 

 of the characteristic minerals of erupted lavas appears to be com- 

 pletely proved. The zeolites, however, occurring in the cavities of 

 lava that has been exposed to the action of water since it was erupted, 

 contain no glass- or stone-cavities, but a few fluid- cavities, as if de- 

 posited very slowly from solution in water. The best examples I 

 have met with are in the Arragonite in the lava of Vesuvius, which 

 have the vacuities equal to about y^-th of the fluid, corresponding to 

 a temperature of 160° C. (320° F.). 



Precisely the same conclusions apply to far more ancient trappean 

 rocks. The augite in some of the basaltic rocks of Scotland has the 

 same characteristic structure as that in the modern lavas of Vesuvius. 

 A very good example of a glass-cavity is shown by fig. 73, containing 

 a bubble and many small crystals deposited on the sides of the 

 cavity. In the case shown by fig. 74, many most distinct crystals 

 have been formed on the sides, but it contains no bubble, whilst 

 sometimes, as fig. 7o, there is a bubble but no crystals. In the felspar 

 of a porphyritic greenstone from Arthur's Seat near Edinburgh^ 

 there occur many stone-cavities ; but, like the felspar itself, they 

 have undergone a great amount of alteration by the subsequent 

 action of water. Fig. 76 is much like some of the cavities in leucite, 

 whilst that shown by fig. 77 evidently contained a bubble like a glass- 

 cavity, but it has been filled with the chloritic mineral that has been 

 introduced by water into nearly all parts of the rock. In fact the 

 microscope clearly shows that the amount of alteration effected by 

 the action of water on these ancient volcanic rocks is very much 

 more than is generally supposed ; and rocks, which to the naked 

 eye appear to contain only two or three minerals, are seen to be 

 made up of ten or twelve. Some of these are the igneous minerals 

 containing glass- or stone-cavities, and others are zeolitic minerals 

 containing fluid- cavities, which indicate that they have been deposited 

 from more or less heated water. The characteristic structure of the 

 minerals of which ancient trappean rocks are composed is, therefore, 

 so analogous to, or even identical with, that of the constituents of 

 modern lavas, that the purely igneous origin of these ancient lavas 

 appears to me to be completely established ; but, at the same time, 

 their present aspect is often to a very great extent due to the sub- 

 sequent action of water. In fact they have frequently been as much 

 metamorphosed by water as some stratified rocks have been by heat. 

 The production of zeolites, by the action of the thermal springs at 

 Plombieres on the ancient masonry, strongly confirms these deduc- 

 tions. (Daubree, Annales des Mines, 5 e serie, t. xii. p. 289, and 

 xiii. p. 227.) 



