366 A.J. Bull—A Hypothesis of Mowntain Building. 
Should a relatively cooler portion of the asthenosphere be situated 
between two hotter portions, all being at about the same level, 
then, when movement commenced, there would be two opposing 
currents dragging the under surface of the crust, which would be 
crumpled along a line perpendicular to the currents. There would 
thus arise a line of folding above the region where the asthenosphere 
was sinking, and the crust outside the moving regions would be in 
tension. Now the folded mass, being lighter than the asthenosphere, 
would float in it and project above the general level of the crust. 
It would thus be a mountain chain with light roots, and denudation 
of the protruding mass would cause a further rise, for as many of the 
cracks and folds would be approximately vertical, isostatic adjust- 
ment would be relatively easy, because the broken crust would have 
little strength, and this might continue until denudation had 
removed the mountain chain and only traces remained, as in 
Devonshire. 
Away from the centre of folding, the underdrag of the slowly 
moving material would make its effects felt in various ways. Where 
there existed old lines of folding there would be a tendency to 
accentuate these, and to increase any want of symmetry, the 
steeper limbs of the anticlines indicating the direction of approach. 
For instance, the Weald, Purbeck, and Ridgeway anticlines have 
their steepest dips on the north or north-west, being in agreement 
with the idea of an undercurrent from north to south, i.e. towards 
the Alpine axis of folding. The effect of the underdrag on rocks 
that were brittle and without a thick cover might well produce 
movements along numerous slide planes, such as have occurred in 
Scandinavia. 
In this connexion I desire to draw attention to a remark made by 
Mr. Bernard Smith before the Geological Society on the 23rd June, 
1920, to the effect that he and Mr. C. B. Wedd had come to the 
conclusion that certain structures in North Wales had been produced 
by the deeper seated rocks moving first and dragging those above. 
This hypothesis furnishes a possible explanation of the following:— 
(a) The intense folding of mountains along well-defined lines 
while rocks sometimes at no great distance show no indication 
of compression, and also the fact that the lower beds are usually 
more distorted than the upper. 
(6) The folding often follows a long period of deposition. 
(c) Old axes of folding or “lines ot weakness” are often 
accentuated. 
(d) The formation of fiords and fissure eruptions. 
(e) The change of type of magma erupted in a particular district 
after long geological intervals, as on the Peruvian coast and.in the 
Great Rift Valley. 
(f) The formation of gneisses and schists from plutonic and other 
rocks, 
The hypothesis suggests a source or energy ample for the 
