Ge'iHe — Volcanic Rocks of Great Britain. 4G7 



sent the submarine lava flows of the time. Sometimes they still 

 preserve the slaggy vesicular character which marked their surface 

 when the melted rock was in a state of motion along the sea bottom. 

 By this and other evidence of a like tendency we learn the existence 

 and position of true submarine volcanos during the lower Silurian 

 period in Wales.' 



Northwards, in the Lake District, Professor Sedgwick has found 

 similar proofs of volcanic action among the lower Silurian rocks of 

 that region, and those rocks are now being worked out in detail by 

 Mr. Aveline and his colleagues of the Geological Survey. 



No very distinct traces of contemporaneous volcanic activity have 

 yet been detected among rocks of this age in Scotland. 



Among the lower Silurian rocks of the south-east of Ireland beds 

 of ash and felstone are interstratified, resembling in general character 

 and mode of occurrence those of Wales, but on a much smaller scale. 

 It has been observed that the Silurian fossils of that region occur 

 only in the upper part of the series in the neighbourhood of the trap 

 rocks and calcareous bands.^ 



Upper Silurian. — In Wales volcanic action does not appear to 

 have outlasted the lower Silurian period, but in the south-west of 

 Ireland, among the headlands of Kerry, massive sheets of ash are 

 intercalated in grits and slates, which from their fossils have been 

 assigned to the age of the Wenlock series.^ 



Old Bed Sandstone. — The Old Eed Sandstone of the southern half 

 of Scotland abounds in igneous rocks, from the base of the series to 

 the top. In its lower band lie the chains of the Sidlaw and Ochil 

 Hills, and many detached masses scattered over the lowlands along 

 the southern flank of the Grampians. These are composed of differ- 

 ent felstones and porphyrites, with interbedded sheets of tuff, trap- 

 pean conglomerate, and sandstone, stretching in the Ochil and 

 Sidlaw range for sixty or seventy miles, and rising here and there 

 to heights of 2,000 feet above the level of the sea. This group of 

 hills contains some of the thickest masses of trappean rock in the 

 country. In what seems to be a middle portion of the formation 

 comes the group of the Pentland HiUs, consisting of long massive 

 beds of trap, like the different varieties in the Ochils, with intercala- 

 tions of tuff, conglomerate, and sandstone, the whole reaching a 

 thickness of fully 5,000 feet.* 



In Ireland, also, the Old Eed Sandstone furnishes evidence of 

 active volcanic vents. Among the picturesque glens and cliffs of the 



^ See Murchison, " Siluria," p. 83. Eamsay, Descriptive Catalogue of Eock 

 Specimens in Jermyn Street Museum, 3rd Edit., p. 8. Mem. Geological Survey, 

 vol. iii. p. 21 et passim. 



2 Jukes, Manual, p. 454. See also Memoirs of Geol. Surv. Ireland, Explan. to 

 Sheets 102, 111, 147, 167. 



3 Mem. Geol. Surv., Ireland. Explanations to Sheet 160, etc., p. 21. 



* In the Upper Old Red Sandstone of Scotland proofs of volcanic activity remain 

 to be gathered. The chain of the Pentland Hills which I formerly regarded as 

 belonging to the upper member of the formation, I have since found to be covered 

 uncouformably by it, while the chain of the Campsie, Kilpatrick, and Renfrewshire 

 Hills seems to belong wholly, or at least in great measure, to the lower part of tlie 

 Carboniferous series. 



