ANNIVERSARY ADDRESS OF THE PRESIDENT. Hx 



that these facts can be completely traced in the compass of half a 

 mile. In the mountain of Black Comb, nearly in the southward 

 prolongation of this ridge, and in the lower strata (Skiddaw slate), 

 are seen bands or dykes of granite (" elvany "), Ipng in the slate 

 series, and apparently, at least for short distances, parallel to the 

 strata. The strata are partly altered to what is often called " Jflinty 

 slate," a change which amounts to little more than loss of colour, 

 and gain in hardness. 



The general conclusion to which Mr. Marshall has been con- 

 ducted in the course of his long study and intimate knowledge of 

 the Lake Mountains is thus expressed : — The phsenomena observed 

 may be best explained by the supposition that the whole series of 

 rocks, granites included, are metamorphic sedimentary strata in 

 situ, or in their natural order of position, and that the slaty rocks 

 alternating with the porphyries are to be accounted for on the sup- 

 position that they are, by chemical composition, less fusible, less 

 easily acted upon by heat, than the porphpitic beds, and have there- 

 fore been only hardened, retaining the cleavage and stratified 

 structure, whilst the more fusible rocks have been changed into 

 porphyries. 



This supposed original inequality in the degree of fusibility has 

 been in some degree submitted to the test of experiment : portions 

 of the Skiddaw slate, green slate, and porphyritic bands, in powder, 

 have been placed in crucibles and gradually heated. At a good red 

 heat the porphyry puffed up, fused, and ran over the edge of the 

 crucible, in the shape of a brown glassy slag. At a white heat the 

 Skiddaw slate fused into a grey glassy slag ; and lastly, at a strong- 

 white heat the green roofing-slate also fused into a black glassy slag. 

 Thus the slate -rocks appear to be decidedly less fusible — ^less easily 

 acted upon by heat than the porphyries. 



The same result was obtained in fusing fragments of Skiddaw 

 slate, Skiddaw granite, and porphyry, in a common reverberatory 

 furnace : a white heat was required to fuse them, but the granite 

 and porphyry melted much more readily than the clay-slate. 



When pressui'e was employed to consolidate the powders of gra- 

 nite, Skiddaw slate, and porphyry, in strong iron tubes, and the 

 compressed mass was secured by screwing down, the melting occurred 

 in each case at a lower heat with pressure than without: a red 

 heat was sufficient to fuse them ; when slowly cooled they resumed 

 a stony texture, and did not resemble the glassy slags produced by 

 fusion without pressure. 



A large mass of the syenite of Charnwood Forest was melted in a 

 reverberatory furnace, and slowly cooled ; it showed, as in the well- 

 known trials by Mr. G. Watt on the basalt of Rowley Rag, every 

 gradation of texture, from that of glassy slag to stony granular and 

 even porphyritic structure. 



By exposing the glassy slag to heat below fusion for two or three 

 weeks, a stony texture is found to be induced. 



It is much to be wished that experiments of this kind may be 

 repeated, under pressure, and, if practicable, in presence of moisture. 



