556 REPOR1—1904. 
mountain ranges of the ancient Thetis. Contraction and this deformation acting 
together might be found to supply the explanation required. A decisive solution, 
however, could hardly be looked for at present ; many physical data, especially the 
value of the coefficient of expansion for solid bodies near their critical fusion point, 
were not sufficiently known, and great authorities were ranged on opposite sides ; 
at the same time, investigators like Joly and Barus were making important addi- 
tions to our knowledge, and further advances might he hopefully looked for in the 
near future. 
Professor J. F. Brake considered it more important to determine, in the first 
instance, the proximate causes of earth movements rather than the ultimate causes, 
which must be more or less speculative. Thus the thrust-planes of the N.-W. 
Highlands had inclinations pretty uniformly bringing upwards the overthrust 
materials, only the small minor thrusts bending down towards the end, as though 
they were locally stopped in their motion. This motion was from the east, where 
there existed a broad area of crystalline rock. These indications suggested the 
former existence of a mountainous region in this direction, which had afterwards 
sunk by its own weight and forced out the strata to the west, carrying with it 
portions of its own base, the yielding of which was the immediate cause of the 
motion. This, however, involved no important folding, and thus supported rather 
the ideas of Professor Rothpletz than those of Professor Heim. Folding, in fact, 
had, in the speaker’s opinion, been much exaggerated; there was plenty of it, but 
too wide effects had been attributed to it. ‘The speaker had never seen ‘isoclinal’ 
strata in the sense often implied, and never hoped to, for such folding was physically 
impossible, involving as it did the absolute destruction of the central layers. At 
most the structure might be called p/estoclinal; and in this case the centre of the 
fold should always be somewhere recognisable, as it occurred throughout. If no 
indication could be found it argued that a fold was absent. There were also 
local causes of folding, of which an instance was quoted in the valley crossed by 
the excavation for dams made in carrying out the Derwent Valley water-scheme. 
Here the two sides of the valley showed uniform horizontal strata in the bedded 
Pendleside series ; but where the stream was crossed a sharp uplift on both sides 
was seen in the section, while the underlying strata were said to be still undis- 
turbed ; thus indicating that the cause of the uplift originated at the stream, and 
was probably the relief from local pressure dug to the excavation of the valley, 
and might be called, therefore, a kind of earth-creep. 
Professor A. Rorupnetz said: I have had much pleasure in listening to the 
clear exposition that Mr. Horne has given us of the overthrusts and earth-crush 
movements in the Scottish Highlands, and in seeing the great importance given to 
these subjects by this meeting. It is more than twenty-five years ago that I had 
the first opportunity of studying an overthrust. At that time nobody cared for 
such things, and the text-books of geology hardly recognised the word at all. 
The next overthrust I found in the Alps twenty years ago, but I had to defend 
it against almost everyone. So you may understand my delight when I came 
to the meeting of this Association at Nottingham, and had the good fortune to 
find there those geologists who had worked out so carefully the overthrusts of 
the Highlands, and to accompany them into that district. 
But now opinions have changed so entirely that nobody in this room will be 
found to deny the existence of overthrusts. 
Although I have now worked twenty-five years at the subject I am still very 
doubtful about overthrusts, because we do not yet know many of their essential 
features. How, for instance, do these wide and long overthrust-planes come to 
an end laterally? That we cannot see in the Highlands; but in the Alps I have 
found two great overthrust-planes which strike from §. toN. at a place where the 
strike of the folds is E. to W. So the shortening in one direction took place by 
folding, while the shortening in a direction at right angles to that was by over- 
thrust. Following those overthrust-planes to the north and south, we see that before 
reaching the boundaries of the Alps they simultaneously and suddenly turn east- 
ward and lose their horizontal character, They become longitudinal and more or 
