586 MR. G. W. LAMPLT7GH ON THE [Nov. 1 895, 



on so extensive a scale. And since I know that it is not in every 

 section that the true nature of the structure becomes apparent, 

 there seems much likelihood, that ' conglomerates ' of this origin 

 have not always been recognized, as such. 



In the highly disturbed Devonian rocks of the south-west of 

 England Mr. J. E. Marr has described 1 the production on a smaE 

 scale of separate ' eyes ' of limestone and of grit by the folding and 

 breaking-up of thin bands, exactly as in the Manx strata. 



It is certain that similar brecciation occurs on an extensive 

 scale in some parts of Ireland. It has been described in the Cam- 

 brian rocks of the north side of Howth by Prof. Sollas, 2 and in 

 the metamorphic rocks of South-west Donegal by Mr. J. R. Kilroe. 3 

 In the last-mentioned instance, indeed, there seems to have been 

 more fluxion-movement and less actual brecciation than in the 

 Manx rocks ; but in other respects there is close similarity, even to 

 the occurrence of two separate periods of earth-movement with an 

 intervening stage during which certain igneous rocks were injected. 

 It is important to notice, however, that while the sheared rocks 

 of Donegal have been highly metamorphosed there is comparatively 

 little mineral alteration in the crush-conglomerates of the Isle of 

 Man (see Mr. Watts's Appendix, p. 591); and it follows that the 

 rending and crushing alone are not sufficient under every circum- 

 stance to bring about extensive metamorphism. 4 



1 am also permitted to state that Messrs. Marr and BZarker have 

 found during the course of their work among the Ordovician rocks 

 of the Lake District, that in certain localities the strata have been 

 extensively broken up by earth-movements, as in the Isle of Man,, 

 though under somewhat different conditions. 



[Since this paper was read my attention has been kindly called 

 to the following references to the subject, previous to those men- 

 tioned above, which had been overlooked. 



In discussing, in 1877, the supposed unconformity at Llanberis,- 

 Prof. T. M C K. Hughes remarks 5 : — ' The coarser and more sandy beds 

 were crumpled and contorted, while the finer beds were compressed 

 and cleaved, and, being thrust into the folds of the harder beds,, 

 gave the appearance of a denuded jagged edge of slaty rock.' 



In a paper read at the same time Dr. H. Hicks says 6 : — ' The 

 curious change in the rocks which he has taken for an unconformity 

 is probably, as suggested by Prof. Hughes, only an appearance due 

 to rock-structure, not uncommon in beds of unequal texture and 



1 ' On some Effects of Pressure on the Devonian Sedimentary Rocks of 

 North Devon,' Geol. Mag. 1888, p. 218. 



2 Proc. Geol. Assoc, vol. xiii. (1893) pp. 92-93 & p. 170. 



3 Mem. Geol. Surv. Ireland, Expl. Sheets 22,. 23, 30, & 31 (S.W. Donegal), 

 1891, pp. 13-42. [See also Mr. McHenry on the Portraine and Balbriggan 

 areas in the discussion to this paper, p. 598.] 



4 Is it not possible that the time-element may in this respect be a factor of 

 prime importance, and that where the crushing has gone on very slowly there 

 has been a concurrent dissipation of the generated heat ? 



5 Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 143. G Ibid. p. 150. 



