Vol. 51.] CKTTSH-CONGLOMEKATES OF THE ISLE OF MAN. 581 



banded portions of the flaggy slates and grits above described. In 

 them every plication and shear-plane is brought out with the 

 greatest distinctness, so long as any vestige of the original bedding 

 remains. Consequently, in such beds, where the exposures are at 

 all satisfactory, tbe making of the crush-conglomerates can be 

 followed from the first simple folding of the strata to their final 

 rending asunder and redistribution. If the layers had been all of 

 one colour and composition, I can conceive that the brecciation 

 might have taken place, and yet after the impression of the later 

 cleavage have been quite imperceptible to the observer. Indeed, in 

 examining weathered faces of the almost homogeneous Barrule 

 JSlates, in which definite phacoids are sometimes marked out by 

 shear-planes, it is permissible to surmise the existence of such 

 masked brecciation. But usually a closer search will reveal some 

 faint signs of distorted stripe in adjacent crags, sufficient to show 

 that if such hidden brecciation do exist it is not very widespread. 

 And I think we are safe in concluding that the structure is- 

 developed on a large scale only in rocks of a definite peno- 

 logical type, and in them only under definite conditions. 



In tracing the banded strata towards the area of brecciation, we 

 may note in them many features foreshadowing their impending 

 disruption (see Mr. Watts's Appendix, p. 589). The planes of 

 strain-slip become more distinct, and their effect upon the bedding 

 more important. Thus, for instance, in a stretched portion of the 

 rock, as illustrated in fig. 2 (p. 568), while the slaty layers still retain 

 their continuity, the less plastic sandy bands are seen to be cut into 

 segments by the strain-slip planes, and these segments are dragged 

 apart, forming a string of len tides. 



A similar effect, though differently brought about, is produced 

 where the rock has been pressed and packed into a narrower com- 

 pass, as in fig. 3 (p. 570), where amid a general thickening of the 

 slaty layers the gritty seams have had their individual segments 

 pushed more or less edgewise, and even an occasional fragment 

 pinched clear away from the bedding-plane. 



In other instances there has been along each strain-slip plane an 

 oblique shift, or small step-fault in the grit-band, so that these 

 layers assume a cable-like aspect, and the surfaces look as if deeply 

 ripple-marked, as in the example illustrated in fig. 4 (p. 571). 



In other cases, though more rarely, instead of this simple sliding 

 of the individual fragments one over the other, there seems to have 

 been a partial rotation of each upon its axis, probably as the com- 

 bined result of end and side pressures, as illustrated in fig. 5 (p. 571). 



It is clear that in any of the above cases a slightly greater dis- 

 placement of the relative position of the fragments would have 

 annihilated the original bedding-planes and produced a crush-con- 

 glomerate. And that this is what has actually taken place is fully 

 proved by many sections in which all the different stages of the 

 process stand revealed : as in the crag shown, by wav of example, 

 in fig. 11 (p. 582). 



In a few places the banded rocks, either from some peculiarity 

 Q.J.G.S. No. 204. 2t 



