TRANSACTIONS OF SECTION C. 5517) 
less vertical fault-planes, which we may follow to the east end of the Alps. So 
they enclose an enormous mass of mountains, which have moved from E. to W. 
along these longitudinal fault-planes like a car on the rails. 
Now, are the particular features of these Alpine overthrusts of general 
application? It may beso or not. Nothing but further field-work can clear up 
this point. 
Of course the forces which made these overthrusts must have been enormous, 
and there is naturally a great tendency to attempt to calculate their origin. 
Field geologists are not so well fitted for doing this, because they have too 
great a knowledge of detail, and are too much puzzled by what they have not yet 
fully elaborated. It is easier for those who are mainly occupied in working at 
theory and who are not disturbed by detail. They can more readily accept the 
simple assumptions which are wanted for mathematical analysis. 
We have just seen, however, that in theory, too, there are many difficulties and 
much difference of opinion; and so I do not think that questions of the nature and 
origin of overthrusts will be solved so quickly as Professor Sollas hopes and 
anticipates. 
Professor Boyp Dawxrns called attention to the foldings and faults caused by 
the relaxation of pressure at the bottom of valleys carved out of the elastic shales 
and thin sandstones of the Yoredale and Millstone Grit rocks, which he had 
observed in the upper valleys of the Derwent and of the Don during the construc- 
tion of reservoirs. ‘lhe folded and faulted strata extend to a depth of from 
40 to 120 feet from the surface, and rest on undisturbed shales and sandstones. 
They are due to the lateral pressure of the sides acting on the valley, from which 
the counterpoise of rock has been denuded away, and are analogous to the ‘ creeps’ 
in coal-workings. We must, therefore, add the relaxation of pressure to the causes 
of folding and faulting recognised in geological theory. 
Professor Joun Minne remarked that the number of worlds which had been 
invented by geologists, physicists, and others exceeded the number which they 
had heard about in their youth. New worlds had even been invented whilst he 
had been in the room, and to them he ventured to add another—a world that would 
meet the requirements of the seismologist. The world which they required was 
one which would convey earthquake waves with a velocity which was nearly 
uniform along chords corresponding to arcs greater than 30°. In other words, 
a fairly homogeneous nucleus was required, and this nucleus should have a 
‘rigidity about twice that of steel. Such a world would not be far removed 
from the one suggested by Wiechert, which was in agreement with the require- 
ments of astronomy and geodesy. In other respects it resembled the world of 
Arrhenius, 
Professor Percy F, Kenpatt said that he had no intention of discussing the 
general question of the causes of earth movements, but merely desired to make 
one observation upon the remarks of Professor Sollas, who had pointed out that 
the continental margins were the seat of the most important mountain-making 
movements. 
Upon the hypothesis that there was a critical zone at which temperature would 
impart a viscosity to rocks, he thought the sub-oceanic margins might, for another 
reason than that suggested by Professor Sollas, be the regions where the yielding 
of the crust would take place. 
The accumulation of a great thickness of sediment, such as took place in proxi- 
mity to coast-lines in a sinking area, would have the effect of producing a rise of 
the ge-isotherms and of the critical zone. If, then, a movement of accommodation 
were initiated, there would he above the critical zone a region of relative weak- 
ness, where the new-formed and imperfectly consolidated sediments constituted 
perhaps two or three miles of the superincumbent crust, while the continental 
mass would be relatively stiffer. The yielding and crumpling would, theretore, be 
localised in this belt. 
