Wynne — Presidential Address. 293 



formed into flat ribands," showing that the rocks have been not 

 only compressed but stretched. 



To such an extent has this deforming metamorphism veiled the 

 original characteristics of one formation, that the various consecutive 

 movements affecting it have become the most convenient means to 

 use in treating of it chronologically, and we further learn that a 

 whole rock group of now uniform character may have been recon- 

 structed from different rocks of different ages and of diverse kinds, 

 their identity being more or less completely lost in the process of 

 alteration. Again, one widely-extending system of induced pres- 

 sure-foliation may be superseded by another of later date, entirely 

 or partially obliterating the first, a result that can scarcely be 

 thought improbable when we find it stated that in certain beds of 

 mylonitic structure the molecular movements resulting from pressure 

 have caused their particles to flow : within the rock. 



While it appears that these thrust-planes may be dislocated by 

 the ordinary shifts of orthodox faulting, often present in the same 

 sections, they seem to be, in a general way, distinguishable from 

 fault planes by their displacements being approximately lateral, 

 the throw of faults being usually more or less nearly vertical, and 

 besides, they are further differentiated by the circumstance that 

 faults have no particularly essential connexion with the metamor- 

 phism of the beds which they traverse. 



Faults upon a grand scale with displacements of thousands of 

 feet or a mile are recorded 2 as occurring in different parts of the 

 Highlands, yet even these enormous dislocations become dwarfed 

 in comparison with the horizontal transplacements of mountain 

 masses' for several miles, 3 movements which must certainly rank 

 amongst the most stupendous revelations of structural geology. 



As a prelude to the discovery of these thrust-movements, our 

 notice was directed to marked inversion or overfolding of strata in 

 connexion with several of the world's greater mountain chains, as 

 well as in the Highlauds, this feature being relied on to account 

 for superposition of more highly altered upon less or quite unaltered 

 rocks. 



1 Lap worth, Page and Lap worth's Geology. 



2 Judd, Address to Geol. Sec. Brit. Assoc, Aherdeen. 



3 Recent work, Geol. Survey, Q. J. G. S., L., 1888, p. 429. 



