Mr. keir on the 
54 ° ' 
obferved, that the prifmatic bafaltes of Auvergne is actu- 
ally a continuation, and generally the termination, of a 
current of lava . 
3. Although the variety of the forms of the cryftals, 
in the fame kinds of glafs, and even in the fame piece 
of glafs, which has been already remarked, iufficiently 
fhews the uncertainty of any inference drawn from a 
iimilarity of fhape; yet it may not be improper to mark 
the analogy, in this refpeCt, between the bafaltic and 
vitreous cryftals. The columnar or prifmatic form is 
known to appear moft generally in the cryftallized ba- 
faltes. Of this form alfo are evidently the cryftals repre- 
fented by fig. 5. The femi-columnar, vitreous cryftals, 
fig. 9. feem to be analogous to the no lefs lingular bafal- 
tic femi-columns obferved in the Giant’s Caufeway by 
Bp. pocock (Phil. Tranf. vol. XLVIII.); which, he fays, 
were exactly like hexagonal columns cut in two. M. 
desmarets has obferved, in the province of Auvergne, 
great quantities of fpherical and ellipfoid bafaltics con- 
cretions, which were formed of polygonal columns, 
rather pyramidal than prifmatic, converging from the 
circumference to the center. Thefe feem to be perfectly 
analogous to the vitreous globular concretions which 
have been above obferved to be compofed of oblong 
cryftals, arranged in a fimilar manner. The fame author 
alfo obferved, in the fame province, regularly -fliapcd 
tables or plates of bafaltes; of which, he fays, affem- 
blages were accumulated in all directions. We have 
fhewn x that the cryftals reprefented by fig. 1 . 2. 3. and 4. 
are 
