C.—GEOLOGY. 73 
In 1812, when he was President of the Royal Society of Edinburgh, 
he read a paper to the Society on ‘ The Contortions of the Strata.’ He 
‘described the Silurian rocks of the Berwickshire coast as having been 
‘thrown into folds, a great part of which had been removed by denuda- 
tion. Similar phenomena had been noted in many other places, and 
“many explanations had been offered to account for the tilted, bent, 
‘upturned, and distorted rocks. Some geologists held they had been 
‘deposited in that position, others that they had been upheaved by earth- 
quakes, or let down by subsidences. — Hall’s explanation was very 
simple—the rocks had been affected by lateral pressure. With.some 
pieces of cloth and a door ‘ which happened to be off its hinges,’ and a 
few stones to act as weights, he was able to reproduce the ‘ contortions 
‘of strata ’ experimentally with great perfection. The whole proceeding 
was so simple that one is reminded of Columbus and the egg. Yet if 
we are to judge by the discussions in contemporary literature, folding 
of strata had not yet been recognised, and this suggestion was revolu- 
tionary. It contains the germ of many theories of mountain building ; 
“no one now doubts that lateral pressure is one of the most powerful 
agencies in the disturbance of the earth’s crust and the production 
of many special types of rocks. In no district is this better exemplified 
than in the North-West Highlands of Scotland. 
As the source of lateral pressure Hall suggested that igneous in- 
trusions making their way upwards might force asunder the -adjacent 
rocks. This would not now be generally accepted, but of course it was 
an explanation very likely to occur to a Huttonian. It seems to have 
‘been about forty years later that Elie de Beaumont and his school 
brought forward the hypothesis that secular contraction of the earth’s 
erust might produce lateral compression of rock masses, and might be 
the cause of folding and of mountain building. The rise of modern 
theories of mountain structure probably dates from the investigations of 
H. D. Rogers on the Appalachians. 
Hall’s final contribution to experimental geology appears in a paper 
which he read to the Royal Society of Edinburgh in April 1825, and was 
published in the tenth volume of the ‘ Transactions.’ The title is ‘ On 
the Consolidation of the Strata of the Earth.’ Modern geology by that 
time had made great progress, and many of the controversies of Hall’s 
early years had been settled. But Hall remained essentially a Huttonian 
in his belief in the efficacy of plutonic heat. He aimed at showing 
that submarine intrusions would consolidate loose overlying beds of 
d into firm sandstone. For this purpose he took salt water, or 
oncentrated brine, and heated it in crucibles or iron vessels containing 
& quantity of sand. He found that it was possible to make the bottom 
of such an iron vessel red hot, while the brine on top was so cold that 
the hand could be inserted into it. The sand was in some cases con- 
verted into firm coherent mass, no doubt by the action of alkalis at 
a red heat. It is difficult to perceive what such an experiment proves ; 
it may possibly have some bearing on the induration of sandstones by 
contact alteration in the vicinity of intrusive sills, and one of the special 
eases to which Hall refers as suggesting this experiment was the 
hardening of conglomerate by intrusive dykes. What actually 
1921 u 
