Vol. 51.] CRUSH-CONGLOMERATES OF THE ISLE OE MAN. 585 



extensive ' creep ' of material from one part of the fold to another, 

 as shown by the thickening of the layers in certain portions and. 

 their attenuation elsewhere. Subsequently when the folds were 

 closed up and could no longer yield, the pressure gradually over- 

 came the cohesion of the bedding-planes along certain zones of 

 intense shearing, and the folded rocks were broken into fragments. 



Some of the larger fragments in the breccias still retain traces of 

 the initial movements, the slaty inclusions in which the original 

 bedding remains occasionally exhibiting a crumpled structure and 

 also, more rarely, an obscure cleavage, which does not coincide in 

 ■direction with any of the structures of the surrounding matrix. 



At some period after the accomplishment of the brecciation, 

 possibly after an interval of long duration during which the older 

 ■dykes were injected and many of the quartz-veins formed, the 

 pressure again became sufficiently powerful to affect the structure 

 of the rock. In this instance, however, the displacement of the 

 particles took place only within very narrow limits of space ; and its 

 effect was to produce a rude shear-cleavage, affecting the crush- 

 conglomerate equally with the uncrushed rocks and the greenstone- 

 dykes. The force of this later pressure can be best gauged by the 

 condition of the quartz-veins in the brecciated area. These fre- 

 quently occur as torn strips along planes of movement, or as wavy 

 plicated strings ; but always, like the dykes, definitely crossing the 

 crush-conglomerate (see fig. 11, p. 582), and I have sought in vain for 

 any fragments of either in the breccia. This seems sufficient proof 

 that neither dykes nor quartz-veins were in existence when the 

 brecciation-movement took place, and that the disturbances which 

 both exhibit belong to the later period. It is, however, necessarily 

 difficult to differentiate out the work of the several stages thus 

 superimposed, and I do not profess to be able in every case to make 

 the distinction. 



Many later displacements have also undoubtedly affected the 

 area, both in pre-Carboniferous and in post-Carboniferous times ; 

 but these have produced only the ordinary effects of simple faulting, 

 and have no direct bearing upon our present subject. 



VI. Similar Rock-structures elsewhere in Great Britain. 



So far as my somewhat imperfect search has gone, I have not 

 been able to glean much information bearing upon these structures 1 

 from the literature of the older rocks of Great Britain ; and yet 

 have found sufficient to show that, as might have been surmised 

 from the widespread distribution of rocks of similar lithological type 

 to the Skiddaw Slates, similarly folded, there is nothing in any way 

 peculiar to the Isle of Man in the occurrence of these crush-con- 

 glomerates, although it is probable that they are not often displayed 



1 The question of the ' augen-gneisses' is not dealt with, since — although there 

 are certain points of resemblance to the crush-conglomerates — the general 

 characters of the rocks are quite distinct. 



