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AND SHROPSHIRE. 



175 



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100 and 101 the streaked 



and tortuous structure incident to the flowing of viscous 

 substances. The manner in which they are associated with 

 the other rocks is especially well shown in sections Nos. i, 

 4 and 7, in the heights of the Y Garw and Y Glyder-fawr, 

 on Snowdon and Moel wyn, and on Y Dduallt and Aran 

 Mowddwy. They lie perfectly interbedded among the 

 slaty and gritty strata and partake of all their curves, show- 

 ing that, long after the formation of sedimentary and igne- 

 ous rocks they have been disturbed together. (See section 

 i Y Garw and Y Glyder-fawr, and No. 4 Snowdon and 

 Moel-wyn.) Under each bed of felspathic trap the slate or 

 grit is altered, that is to say, it is hardened, and, as it were, 

 porcelainised or baked, and above each lava- bed the stratum 

 is in its ordinary condition, proving that the lava flowed 

 over the under stratum in a hot and melted condition, and 

 that after, or during the process of cooling, the upper sedi- 

 ments were deposited upon it. The strata amid which these 

 felspathic traps lie are charged with the ordinary Lower 

 Silurian fossils (see the horizontal gallery cases below), 

 and this indicates that the Silurian volcanos of the time in 

 this area were partly submarine, or, at all events, that 

 many beds of lava flowed far into the sea. 



Felspathic and calcareous volcanic ashes are asso- 

 ciated with the other rocks. They are coloured vermilion 

 in the sections of a paler hue than the felspathic traps, and 

 when calcareous, or when they contain much ordinary 

 sediment, they are streaked with blue and light purple. 

 The specimens .Nos. 5 to 71 will give an idea of their struc- 





ture. Nos. 5, 9, 18, 21, 26, 31, 33, 69, and 71 

 eminently typical kinds. 



5 shows the bedded character. 

 9 shows the porphyritic character. 

 18 shows the porphyritic character partly decomposed 

 21 shows the porphyritic compact nature of the rock. 



are 



26 shows the porphyritic character 

 associated. 



>/ 



Upper 



Gallery. 



"Wall-case 4. 



