546 ISLE OF MAN. GEOLOGY. SCHIST. 



ferentJy in all parts of the island, on the shores as on 

 the summits of the highest mountains. 



In certain cases the clay slate contains a greater 

 proportion of siliceous matter, which, according to the 

 mode of its disposition, produces numerous other modifi- 

 cations. The whole acquires at times a flinty hardness 

 with an uniform aspect, and, in proportion to the in- 

 crease of the siliceous ingredient, becomes at length con- 

 verted into a quartzose clay slate, a substance well known 

 to accompany ordinary slate in other situations, and 

 not unfrequently confounded with indurated schist, or 

 flinty slate, as it is called. Where the grains of quartz 

 are distinct, it acquires a coarser texture, and thus 

 passes gradually into a graywacke, or at least into a 

 substance which cannot, either by aspect or definition, 

 be distinguished from it. Interspersed grains of mica 

 occasionally diversify these varieties still further, but a 

 sufficient idea of them has already been given. To 

 describe the more extensive masses of graywacke in 

 all their modifications, would be superfluous. 



It is almost equally unnecessary to say that the 

 whole of these schists are every where traversed by 

 veins of quartz. One detached and solitary fact occurred 

 during the examination of the schist, which must not 

 be passed over; although most geologists will now 

 consider the argument which it offers respecting the 

 mechanical deposition of argillaceous schist as superfluous. 

 It was that of a rounded pebble imbedded in the clay 

 slate, at the mouth of the Santon river. This was of 

 an oval shape, evidently rounded by mechanical action, 

 and decomposed on the surface ; consisting of a harder 

 variety of clay slate with a few interspersed grains of 

 hornblende. It has on other occasions been shown that 

 the other two leading members of the primary strata, 

 namely, quartz rock and micaceous schist, sometimes con- 

 tained both fragments and rounded pebbles, of the same 

 or of different substances, imbedded in the finer mass 



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