Cryjiallizations obferved on G/a/s. 539 
of the femi-columnar ones, arranging themfelves round 
a common center like the fpokes of a wheel. The 
globular cryftals in the fpecimen N° 4. have been al- 
ready obferved to confifl of many needle-like cryftals, 
converging to one central point. 
Does not this difeovery, of a property in glafs to cry- 
Ifallize, reflect a high degree of probability on the opi- 
nion, that the great native cryltals of bafaltes , fuch as 
thofe which form the Giant’s Caufeway, or the pillars 
of StafTa, have been produced by the cryftallization of a 
vitreous lava , rendered fluid by the fire of volcanos ? 
This opinion is further confirmed by the following 
eonfiderations. The prifmatic and other regularly- 
fhaped bafaltes have been almoft always found to be 
accompanied with lava , pummice-ftones, and other vef- 
tiges of the fire of the volcanoes, whenever they have 
been carefully examined by intelligent naturalifts, as has 
been fhewn by M. desmarets, in his Memoir on the 
Bafaltes of the province of Auvergne, in France; Mem . 
de P Acad. des Sciences, 1771. Bafaltic columns have 
even been difeovered, according to the fame author, 
among the productions of volcanoes now" exifting, as of 
thofe of Mount Etna and of the Ifle of Bourbon. 
2. The fubftance of which thefe bafaltic makes con- 
filt, is generally of the fame nature and appearance as 
the neighbouring and adjoining lava. It is generally 
compaCt, fufible, and of various degrees of hardnefs, 
probably according to the matters of which the vitreous 
mafs was compounded. M. desmarets has further 
Vol. LX VI. 4 B obferved, 
