Vol. 51.] CRUSH-CONGLOMERATES OF THE ISLE OE MAN. 565 



rock ; and when, as frequently happens, they lie very close together 

 and the inclusions are all of small size, the fragmental character of 

 the mass is not at all evident in ordinary exposures, and only 

 appears when a fresh cross-fracture has been made in it. There is 

 usually an abundant development of iron pyrites in the rock, some- 

 times finely disseminated, and sometimes, as at Cronk Sumark, in 

 cubical crystals from § to | inch in diameter showing very marked 

 signs of crushing and shearing. 



To the unaided eye the rock presents no especial evidence of 

 mineral metamorphism, either in the slaty matrix or in the separate 

 fragments, the neighbouring unbroken rocks frequently exhibiting 

 greater alteration in this respect. Its appearance under the micro- 

 scope is described in Mr. Watts ; s Appendix to this paper. 



Such is the composition of the rock over large areas ; and where 

 the modifications which reveal its true origin are absent, it might 

 readily be mistaken for a highly-sheared rock of originally frag- 

 mental character, though a lithological examination of the included 

 fragments would show at once that it was not a true tuff, while the 

 slaty nature of the matrix would render difficult its explanation as 

 a water-borne conglomerate. If we were to imagine an ancient 

 boulder-clay, made up of Skiddaw-Slate material and thoroughly 

 consolidated and much altered by shearing, we should have a rock 

 very similar to this in appearance. But that it is essentially a 

 ' crush-conglomerate,' due to the breaking-up under pressure of alter- 

 nations of flaggy slate and thin grit-bands, becomes abundantly 

 evident wherever a wide exposure is studied. 



III. The ' Skiddaw Slates ' of the Isle of Man. 



Since very little detailed information regarding the Skiddaw Slates 

 of the Isle of Man has yet been published, it becomes necessary 

 hefore dealing further with the crush-conglomerates to give an 

 outline of the leading features of these rocks. 



The Skiddaw Slates, occupying 200 square miles or four-fifths of 

 the whole Island, consist of a mass of sedimentary strata of unknown 

 thickness, ranging lithologically, by alternations, from coarse-grained 

 grits, through sandy flags, to fine argillaceous shaly slates. They 

 are much cut and interpenetrated by igneous intrusions ; but in 

 spite of some statements to the contrary, 1 no positive proof of 

 contemporaneous volcanic activity has yet been forthcoming. No 

 base to the series is known, and where its superior surface comes in 

 contact with the overlying Carboniferous rocks a very pronounced 

 unconformability is presented. With the exception of Palceochorda 

 and other obscure worm-markings, which are rather plentiful in the 

 sandy beds, the only fossils as yet known are the Dictyonema and 

 the imperfect cast of a trilobite recorded by Mr. Bolton, 2 and these are 

 insufficient to settle the age of the rocks, which may be Lower 



1 Harkness and Nicholson, Quart. Journ. Greol. Soc. vol. xxii. (1866) p. 488 ; 

 H. Bolton, Rep. Brit. Assoc. (Nottingham) 1893, p. 770. 



- Op. supra cit. and Greol. Mag. 1893, p. 29. 

 Q. J.G.S. No. 2U4. 2 s 



