VOL. LXVI.] PHILOSOPHICAL TRANSACTIONS. 107 



Does not tliis discovery, of a property in glass to crystallize, reflect a high 

 degree of probability on the opinion, that the great native crystals of basaltes, 

 such as those which form the Giant's Causeway, or the pillars of StafFa, have 

 been produced by the crystallization of a vitreous lava, rendered fluid by the fire 

 of volcanos ^ This opinion is further confirmed by the following considerations. 

 The prismatic and other regularly-shaped basaltes have been almost always found 

 to be accompanied with lava, pumice-stones, and other vestiges of the fire of 

 the volcanos, whenever they have been carefully examined by intelligent 

 naturalists, as has been shown by M. Desmarets, in his Memoir on the Basaltes 

 of the province of Auvergne, in France; Mem. de I'Acad. des Sciences, 1/71. 

 Basaltic columns have even been discovered, according to the same author, 

 among the productions of volcanos now existing, as of those of Mount Etna 

 and of the Isle of Bourbon. 



2. The substance of which these basaltic masses consist, is generally of the 

 same nature and appearance as the neighbouring and adjoining lava. It is gene- 

 rally compact, fusible, and of various degrees of hardness, probably according 

 to the matters of which the vitreous mass was compounded. M. Desmarets has 

 further observed, that the prismatic basaltes of Auvergne is actually a continua- 

 tion, and generally the termination, of a current of lava. 



3. Though the variety of the forms of the crystals, in the same kinds of 

 glass, and even in the same piece of glass, which has been already remarked, 

 sufficiently shows the uncertainty of any inference drawn from a similarity of 

 shape ; yet it may not be improper to mark the analogy, in this respect, between 

 the basaltic and vitreous crystals. The columnar or prismatic form is known to 

 appear most generally in the crystallized basaltes. Of this form also are 

 evidently the crystals represented by fig. 6. The semi-columnar, vitreous 

 crystals, fig. 10, seem to be analogous to the no less singular basaltic semi- 

 columns observed in the Giant's Causeway by Bishop Pocock, Phil. Trans., 

 vol. 48 ; which he says were exactly like hexagonal columns cut in two. 

 M. Desmarets has observed, in the province of Auvergne, great quantities of 

 spherical and ellipsoid basaltic concretions, which were formed of polygonal 

 columns, rather pyramidal than prismatic, converging from the circumference 

 to the centre. These seem to be perfectly analogous to the vitreous globular 

 concretions which have been above observed to be composed of oblong crystals, 

 arranged in a similar manner. The same author also observed, in the same 

 province, regularly-shaped tables or plates of basaltes; of which, he says, 

 assemblages were accumulated in all directions. We have shown, that the 

 crystals represented by fig. 2, 3, 4, and 5, are really assemblages of plates or 

 tables, disposed in every direction round a common axis. 



Lastly. The stone on which the columns of basaltes generally rest, and which 

 sometimes also is supported by these columns, being of the same nature and 



p 2 



