146 



EXPERIMENTS ON BASALT. 



fied with other rocks, it may be proper to mention certain experi- 

 ments that have been made, to elucidate the formation of basaltic 

 rocks. All trap rocks are fusible, and most of them, after melting, 

 form a blackish-green glass: hence it was inferred, that trap rocks 

 had never been in a state of fusion; for if they had, they would 

 have been rendered vitreous. Sir James Hall, however, reflecting 

 on the long period of refrigeration that vast masses of melted rock 

 would necessarily require, before they were cooled to the common 

 temperature of the earth, was induced to make experiments on lava 

 and basalt ; from which it was ascertained, that if a small portion of 

 liquid lava were suddenly cooled, it formed a black glass, as was 

 well known to be the case with basalt, but if the process of cooling 

 were slow, both melted lava, and basalt became stone. When the 

 glass which had been formed by sudden cooling was melted again, 

 and suffered to cool very gradually, it lost its vitreous character, and 

 was converted into a substance resembling basalt. Mr. Gregory 

 Watt made, in one of his father's furnaces, some experiments on 

 the fusion and refrigeration of basalt, which throws much additional 

 light on the formation of the globular and columnar structure of ba- 

 saltic rocks. He fused seven hundred weight of the Dudley basalt 

 called Rowley ragg, and kept it in the furnace several days after the 

 fire was reduced. It melted into a dark-coloured glass, with less 

 heat than was necessary to melt the same quantity of pig-iron. In 

 this glass, small globules were formed, which afterwards disappeared; 

 and as the cooling proceeded, the mass was changed from a vitreous 

 to a stony substance : other globes were again formed within the 

 stony mass, which continued to enlarge until their sides touched and 

 pressed against each other, by which pressure the globes formed 

 polygonal prisms. If part of the mass were cooled before the glob- 

 ular structure was destroyed, these globes were harder than the sur- 

 rounding stone, and broke in concentric layers. In this manner, th^ 

 balls of basalt and porphyry which fall out of decomposing rocks 

 were probably formed ; they derived their superior hardness from 

 the crystalline arrangement of the particles, when in a melted state. 

 When these globes were enlarged by a continuation of the same pro- 

 cess, they might press on each other, and form prisms. The upper 

 prisms pressing by their weight upon the lower, might form concavi- 

 ties or sockets, into which they would sink, and remain joined to- 

 gether or articulated. Such is frequently the structure of basaltic 

 columns. 



Another experiment, made by Sir James Hall, on the crystalliza- 

 tion of common limestone by heat, and its conversion into marble^ 

 tends to elucidate the effects produced by basaltic rocks, on lime- 

 stone and chalk before mentioned. Dr. Hutton had advanced the 

 opinion, that beds of limestone were formed of the shells and exuviae 

 of marine animals, which had been melted by central fire, and crys- 

 tallized. The first part of this theory respecting the entire forma- 



