1 88 The Founders of Geology LECT. 



to twelve feet broad, run up the old crater-wall. These 

 bands seemed to him at the time "to present only an 

 amusing variety in the history of volcanic eruptions," and, 

 like Dolomieu and Breislak, he looked on them as marking 

 the positions of rents which, formed in the mountain 

 during former volcanic explosions, had been filled in 

 from above by the outflow of lava down the outer fissured 

 surface of the cone. Subsequent reflection, however, led 

 him to reconsider this opinion, and to realize that these 

 " vertical lavas " were " of the utmost consequence in 

 geology, by supplying an intermediate link between the 

 external and subterraneous productions of heat. I now 

 think," he remarks, " that though we judged rightly in 

 believing those lavas to have flowed in crevices, we were 

 mistaken as to their direction ; for instead of flowing down- 

 wards, I am convinced they have flowed upwards, and that 

 the crevices have performed the office of pipes, through 

 which lateral explosions have found a vent." He had 

 observed, also, that the outer margins of some of these 

 dykes, in contact with the surrounding rock, were vitreous, 

 while the central parts presented the ordinary lithoid 

 texture. This difference, he saw, was fully explained by 

 his fusion experiments, the lava having risen in a cold 

 fissure, and having been suddenly chilled along its outer 

 surface, while the inner parts cooled more slowly and took 

 a crystalline structure. 



These observations are of historic interest in the pro- 

 gress of volcanic geology. Hall had sagaciously found the 

 true interpretation of volcanic dykes, and he at once 

 proceeded to apply it to the explanation of the abundant 

 dykes of Scotland. He thus brought to the support of 



