C.—GEOLOGY 59 
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years afterwards the problem was completely solved by Fouqué and 
Lévy, who used a gas furnace and a nitrogen thermometer. They 
found that it was possible to obtain either porphyritic or ophitie struc- 
1 ture by modifying the conditions, and that the minerals had exactly 
the characters of those of the igneous rocks. Some of Hall’s re- 
crystallised dolerites were examined microscopically by Fouqué and 
Lévy, and, as might be expected, they proved to be only partly 
crystallised, showing skeleton crystals of olivine and felspar with 
grains of iron ore in a glassy base. 
Some curious observations made by Hall in his experimental work 
were also confirmed by Fouqué and Lévy. The crystalline whinstones 
- were more difficult to melt than the glasses which were obtained from 
them, and the glass crystallised best when kept for a time at a tem- 
_ perature a little above its softening point. It is not possible to assign 
a definite melting-point to the Scottish whinstones with which Hall 
worked. Many of them contain zeolites, which fuse readily. Minerals 
are also present that decompose on heating, such as calcite, dolomite, 
chlorite, and serpentine. The whole process is very complex, and 
_ probably takes place by several stages not sharply distinct. Similarly 
the glasses cannot be said to have a melting-point. They are really 
super-cooled liquids. A full explanation of what took place in Hall’s 
crucibles cannot be given at the present day, but there is no room 
for doubt that his experiments were good and his inferences accurate. 
His friend Kennedy, who had recently discovered the presence of 
alkalis in igneous rocks, furnished valuable support to Hall’s conclu- 
sions by showing that the chemical composition of whinstone and of 
basalt were substantially identical. 
_ Apparently the results of Hall’s work were not received with 
unmixed approbation. Hutton was distinctly uneasy, and it has been 
suggested that he feared if experimental work turned out unsuc- 
cessful it might bring his theories into discredit. The Wernerians 
frankly scoffed ; they preferred argument to experiment, and the endless 
‘discussion went on. Gregory Watt repeated Hall’s experiments by 
fusing Clee Hill dolerite, a hundredweight or two at a time, in a blast- 
furnace. But there can be no doubt that among those who were not 
already committed to the principles of Werner the new evidence pro- 
duced a strong impression, and helped to widen the circle of Hutton’s 
supporters. 
_ Hall’s most famous experiments were on the effect of heat com- 
bined with pressure on carbonate of lime. The problem was, Can 
powdered chalk be converted into firm limestone or into marble by 
heating it in a confined space? In this case Hutton’s theories were 
in apparent conflict with experimental facts; from general observations 
he held it proved that heat and pressure had consolidated limestones 
and converted them into marbles. It was well known, of course, that 
limestone, when heated in an open vessel, was transformed into quick- 
lime, and Black had shown that the explanation was that carbonic 
acid had been expelled in the form of a gas. 
__ The experiments were begun in 1790, but deferred till 1798 after 
Hutton’s death. Hutton quite openly disapproved of experiments. His 
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