Vol. 51.] CRUSH-CONGLOMERATES OF THE ISLE OF MAN. 583 



these rocks have together suffered intense deformation, it seems 

 not unlikely that the differential movement to which the crush- 

 conglomerates bear testimony has been the result of the different 

 degrees of compressibility of which these two distinct types of rock 

 are capable. On a small scale this difference is clearly illustrated 

 in some of the sections above described, and its effect upon a broad 

 scale must have been very considerable. 



If, under the influence of a powerful lateral thrust, the slates 

 were forced into narrower bounds than the same pressure was able 

 to induce in the grits, a partial rending asunder of the interlocked 

 masses and a shearing of the one over the other must have inevitably 

 followed. Under such conditions I imagine the crush-conglomerates 

 to have been produced, through the breaking-up of the passage-beds, 

 as shown in the accompanying diagram (fig. 12). Except for the 



Fig. 12. — Diagram to illustrate the p>ossible result of strong lateral 

 pressure upon a more compressible argillaceous series, resting 

 on and passing into less compressible sanely rocks. 



a-a (between dotted lines) = space originally occupied by the gritty beds. 

 b-b (between dotted lines) = space originally occupied by the argillaceous beds. 

 A-A = Grits after compression. B-B = Slates after compression. 



0. Zone of shearing and brecciation. 



Note. — The beds are supposed to be driven in from the side indicated 

 by the horizontal arrow. 



presence of these passage-beds, and the absence therefore of a 

 sharp line of demarcation, it is probable that the strain would have 

 found relief in the formation of a great thrust-plane, or a series of 

 such planes. Instead of this, the rending force was distributed 

 through a large thickness of rock, which was thereby shattered. 



The occurrence of the conglomeratic structure at one side only of 

 the stratigraphical axis is an important point which I shall not 

 seek at present fully to discuss. It appears to indicate the existence 

 of some such definite relationship between the general structure of 

 the Island and the brecciation as I have suggested above. On the 

 opposite side of the axis there seems to be a fairly well-defined 

 zone of slates apparently altered, especially in the vicinity of thick 



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