Wynne — Presidential Address. 295 



Scotland, or still further westward by Donegal and Mayo, through 

 a tract which has since frequently, if not continuously, been the 

 theatre of mountain formation growth and decay, and which, 

 though its skeleton alone remains, still presents the finest mountain 

 scenery of the British Isles. 



Regarding the great subterranean movements alluded to, the 

 question will arise : in what way has this crust moving force 

 originated, and how has it been locally applied : was it chiefly or 

 entirely in action during past geological eternities, or is it in pro- 

 gress still ? From the nature of the subject, in order to seek for 

 answers we must wander amongst theories, endeavouring to grasp 

 detailed or definite ideas, which those best acquainted with the 

 physical aspects of the case have only approached, and the most 

 hopeful result will be an arrival at the most probable of pos- 

 sibilities. 



The more strongly we realise that great masses of the earth's 

 crust have travelled bodily past or over one another through miles 

 of space, the more urgent becomes the demand to know how they 

 were afforded room to move, or where the movement begun or 

 ended ? 



We are told that the folding of rocks has only been produced 

 at a certain depth in the crust, beneath which depth the compres- 

 sion usually attributed to contraction becomes exchanged for 

 tension or stretching. 1 This of itself conveys an idea of ex- 

 pansion. 



Quite lately Dr. Ricketts has urged 2 that the tangential pressure 

 resulting from subsidence of portions of the crust is quite inadequate 

 to cause the many folded contortions of strata so commonly observ- 

 able. He adduces a simple experiment in support of this, showing 

 that to bend a horizontal lath 10 feet long into an open arch 6 

 inches in height, would only reduce the limits previously occupied 

 by its ends to the extent of little over f ths of an inch, that is, about 

 the T ^th part of the length of the unbent lath. From the mere 

 recollection of any ordinary section in folded rocks it will be 

 evident that these, flattened out to their original limits, would 

 occupy much more than the T ^oth of the length of the section in 



1 Mr. C. Davison, Phil. Trans. R.S., vol. 178, p. 240; Hull's Physiography, p. 55. 

 2 In the Geol. Mag. for February, 1889. 



