Vol. 63.'] SOUTHERN ORIGIN OF THE SAVOY AND SWISS ALPS. 301 



but are we justified in assuming that a wedge of ordinary rock 

 would have done the same? Web clay, in descending, often 

 makes, as we know, an excellent model of a glacier ; yet the same, 

 when dry, is obdurate to the appeal of gravity. That rocks can be 

 bent, that they become, to a small amount, extensible under very 

 great pressures, is certain ; but the fact that even then rupture is 

 frequent, as thrust-faults bear witness, shows that between a 

 viscous fluid, like cobbler's wax, and a very slightly plastic solid 

 like ordinary rock, 1 there is an interval so wide that if imagination 

 attempts the leap, the rider is in danger of becoming a scientific 

 Curtius. 



A small diagram may help us in forming a clearer picture of the 

 conditions prior to the rise of the present Alps. Let AB represent 

 an arc of the earth's crust (for simplicity supposed smooth) 75 miles 

 in length (fig. 3, p. 300). Taking the radius as 4000 miles, the angle 

 subtended at the centre will be just over a degree ; hence the 

 curvature of AB is so slight, that in a small-scale diagram it 

 may be represented by a straight line. Taking 13,000 feet (say 

 2J miles) to represent the total thickness of the strata from the 

 Trias to the ITysch (inclusive) in this part of the Alps, then the 

 oblong ABCD (drawn to scale) represents a section of the rock-mass 

 which is to be the scene of the flatfolding and overleaping, of which 

 the ultimate result will be to bring points (oc) from the neighbour- 

 hood of BC into that (y) of AD. It may, however, be said that the 

 former line was longer than the latter ; it may have been, but I 

 am not aware that any evidence exists to show that the difference 

 was appreciable at the end of thePlysch epoch — or, in other words, 

 that motion began until weights were placed near the end BC. 

 Let us suppose this to have been done, or at any rate to have 

 become important, in Oligocene and Miocene times. During these 

 deposition undoubtedly occurred, but on both sides of the Alps ; and, 

 in order to find an area on the southern one where the deposits 

 were exceptionally thick, we must cross the Po, or in other words 

 at least double the length of our oblong. Yet even then we are 

 precluded from making full use of the exceptional thickness of 

 the Ligurian Oligocene (nearly 12,000 feet) 2 because, unless the 

 viscosity of the rocks approached rather closely to that of cobbler's 

 wax, the rising Apennines would soon begin to interfere with the 

 transmission of the thrust. The pressure, I submit, would not nearly 

 suffice to set almost the whole mass in motion, as is represented in 

 Prof. Sollas's diagrams. In an ordinary valley, which cuts deep 

 into a plateau formed of horizontal strata, the weight of at least 

 500 feet of rock produces no conspicuous squeezing-out on an under- 

 lying argillaceous bed ; and, when the adit of a coal-mine severs 



1 Suppose that two balls, 3 inches in diameter, one of which is moulded 

 from cobbler's wax, the other chiselled from limestone, be put upon a shelf. 

 The former, in a few hours, will have ' sat down ' into the shape of a bun : the 

 latter will not alter its shape appreciably in three score years. 



. 2 Sir Archibald Geikie, ' Textbook of Geology ' 4th ed. vol. ii (1903) p. 1259. 

 This would double the line AB. 



