296 Scientific Proceedings, Royal Dublin Society. 



excess ; indeed, an excess of half of the compressed sections' 

 length would be more probable, and hence it would appear that if 

 the folding resulted from downward pressure upon arched beds, the 

 conditions would require the arch to have had a height about 

 equal its own span. 



Another method of accounting for the folding and thrusting 

 under notice is suggested by my colleague, Mr. Kilroe 1 (who has 

 been unavoidably prevented from placing his views before you 

 to-night) . 



He suggests that continual submergence of accumulating 

 deposits from superincumbent pressure would of necessity involve 

 attenuation of the lower strata coincident with thickening and 

 folding of the beds beneath other regions, where vertical pressure 

 became reduced, either by bulging, due to contraction of the crust 

 in cooling, or by denudation, or both together ; the space necessary 

 for such thickening or folding being afforded by the upward 

 bulging of areas of least vertical pressure. He adds further 

 that, as Tresca 2 proved, solid bodies flow as if they were viscid 

 under very great pressure, which implies attenuation of the mass 

 when the pressure is greatest ; and he instances the rolling or 

 drawing out of ellipsoidal pebbles into lenticular bands of schist 

 under great pressure, as in the case of the Moberg conglo- 

 merate in Norway described by Reusch. 3 This pressure would be 

 accompanied in the regions affected by the development of heat 

 (moderated by certain conditions) and also by chemical reactions? 

 shown by Spring to have taken place under excessive pressure 

 without the application of external heat. 



To gain an idea of this pressure he takes as an illustration the 

 lower Silurian formation, with an approximate thickness of 20,000 

 feet, whieh would mean 1300 atmospheres, or 10 tons of pressure 

 'to the square inch in its lowest strata. But he supposes this 

 pressure inactive towards producing metamorphism until the 

 disturbance of ' the cosmical equilibrium ; this once disturbed, 

 changes would be initiated which might become manifest in widely 

 separated regions though others escaped metamorphism, in con- 



X J. E. KilroeMS. 



2 Tresca, l'ecoulement des corps solides. 



3 Die Fossilien friihender laystalliniselien ScHeier, pp. 24, 25. 



