1 90 The Founders of Geology LECT. 



influence of great compression must retard the fusion of 

 mineral substances, and retain within them ingredients 

 which, at the ordinary atmospheric pressure above ground, 

 are rapidly volatilized. He thus accounted for the retention 

 of carbonic acid by calcareous rocks, even at such high 

 temperatures as might melt them. Here then was a wide 

 but definite field for experiment, and Hall entered it with 

 the joy of a first pioneer. As soon as he had done with his 

 whinstone fusions, he set to work to construct a set of 

 apparatus that would enable him to subject minerals and 

 rocks to the highest obtainable temperatures in hermetically 

 closed tubes. For six or seven years, he continued his 

 researches, conducting more than 500 ingeniously devised 

 experiments. He enclosed carbonate of lime in firmly 

 secured gun -barrels, in porcelain tubes, in tubes bored 

 through solid iron, and exposed it to the highest tempera- 

 tures he could obtain. 



He was able to fuse the carbonate without the loss of 

 its carbonic acid, thus practically demonstrating the truth 

 of Hutton's contention. He obtained from pounded chalk 

 a substance closely resembling marble. Applying these 

 results to the Huttonian theory, he contended that the 

 effects shown by his experiments must occur also on a 

 great scale at the roots of volcanoes ; that subterranean 

 lavas may melt limestone ; that where the molten rock 

 comes in contact with shell-beds, it may either drive off 

 their carbonic acid or convert them into limestone, accord- 

 ing to the heat of the lava and the depth under which it 

 acts ; and that his experiments enabled him to pronounce 

 under what conditions the one or the other of these effects 

 would be produced. He concluded that having succeeded 



