58 SECTIONAL ADDRESSES 
sions, which were strikingly novel at the time, have been abundantly 
confirmed, 
We obtain a pleasant glimpse of Hall’s life in Berwickshire in the 
account of his visit with Hutton and Playfair to Siccar Point in the 
year 1788. The start was made from Dunglass, where probably the 
party had spent the night. The great conglomerates of the Upper 
Old Red Sandstone of that district had much impressed Hutton. He 
saw in them the evidence of new worlds built out of the ruins of the 
old, with no sign of a beginning and no prospect of an end—a thesis 
which was one of the corner-stoncs of his ‘ Theory of the Earth.” No 
doubt Hall knew or suspected that in the cliff-exposures at Siccar 
Point, where the Old Red rests upon the Silurian, there was evidence 
which would put this dogma to a critical test. 
Hall’s first experiments were begun in the year 1790, his object 
being to ascertain whether crystallisation would take place in a molten 
lava which was allowed to cool slowly. It was generally believed that 
the results of fusion of rocks and earths were in all cases vitreous, but 
glassmakers knew that if glass was very slowly cooled, as sometimes 
happened when a glass furnace burst, the whole mass assumed a stony 
appearance. An instance of this had come under Hall’s notice in a 
glassworks in Leith, and its application to geology was clear. Hutton 
taught that even such highly crystalline rocks as granite had been com- 
pletely fused at the time of their injection, and their coarse crystallisa- 
tion was mainly due to slow cooling. 
For the purpose of his experiments Hall selected certain whin- 
stones of the neighbourhood of Edinburgh, such as the dolerites of the 
Dean, Salisbury Crags, Edinburgh Castle, the summit of Arthur’s 
Seat, and Duddingston; but he also used lava from Vesuvius, Htna, 
and Iceland. He made choice of graphite crucibles, and conducted his 
experiments in the reverberatory furnace of an ironfoundry belonging 
to Mr. Barker. As had been shown by Spallanzani, to whose exper'- 
ments Hall does not refer, lavas are easily fusible under these con- 
ditions. Hall had no difficulty in melting the whinstones and obtaining 
completely glassy products by rapid cooling. He now proceeded to 
crystallise the glass by melting it again, transferring it from the 
furnace to a large open fire, where it was kept surrounded by burning 
coals for many hours, and thereafter very slowly cooled by allowing 
the fire to die out. He succeeded in obtaining a stony mass in which 
crystals of felspar and other minerals could be clearly seen. Some of 
his specimens were considered to be very similar in appearance to the 
dolerites on which his experiments were made. 
The only means of measuring furnace temperatures available at 
that time were the pyrometers which had recently been invented by 
Wedgwood. Hall found that a temperature of 28 to 30 Wedgwood 
yielded satisfactory results. This seems to be about the melting-point 
of copper, approximately 1000° C. 
Whether by design or accident, Hall chose for his experiments 
precisely the rocks which were most suitable for his purpose. If 
granite had been selected no definite results would have been obtained. 
De Saussure had already made fusion experiments on granite. Ninety 
