Joseph Nolan — Volcanic District of Slieve Gidlion, 449 



the more fusible minerals, since the difference in the specific gravities 

 would probably be small compared with the power of the eruptive 

 force." And in a note he adds : — ' The chemist is reminded of the 

 fact, that if a mixture of metals, as, for instance, tin, bismuth, and 

 lead, be melted, they will, as the mixture cools, have a tendency to 

 solidify and crystallize out separately as the temperature of the mass 

 reaches their respective melting points. This constitutes a great 

 difiiculty in large bronze castings." (Student's Manual of Geology, 

 p. 92.) 



I have now briefly traced the volcanic history of this remarkable 

 mountain. The first igneous disturbances were apparently those 

 connected with the formation of granite through metamorphism of 

 the Lower Silurian sedimentary strata. This action was probably not 

 unattended with some manifestations of a volcanic nature, all traces 

 of which, however, were probably removed even before the period of 

 the Upper Silurian formation ; denudation, assisted by other physical 

 agencies, having, as we have seen from the example in the West of 

 Ireland, removed all the superincumbent strata and exposed the 

 deep-seated metamorphic rocks. At a subsequent period, probably 

 about the close of the Palaeozoic epoch, volcanic disturbances recom- 

 menced, and the massive dolerites and granitoid rocks were protruded 

 into, but not through, the granitic area. Simultaneously with this 

 intrusion, or a little later, a lateral mass of molten rock was projected 

 sufficiently near the surface to overcome the pressure of the over- 

 lying rocks, throughout a large area, but, partly on account of the 

 distance from the volcanic focus, in conjunction with other causes, 

 the eruption, though violent, was of a very temporary character, and 

 the great volume of displaced materials falling back into the gulf 

 still further checked its activity, so that the intumescent mass was 

 never developed into lava or scorige. 



Perhaps the most instructive feature in the physical history of 

 this district is the evidence it affords of the intimate connexion 

 between the granitic and the volcanic rocks. Prof. Jukes writes : — 

 " If we could follow any actual lava-stream to its source in the bowels 

 of the earth, we should, in all probability, be able to mark in its 

 course every gradation from cinder or pumice to actual granite." 

 (Student's Manual of Geology, p. 93.) Prof. Judd has recently 

 demonstrated that in some of the western isles of Scotland this con- 

 nexion can be traced in all its stages, and the ultimate development 

 of the granitic rocks into contemporaneous lava flows, with their 

 accompanying scorise, ash, and lapilli, is there fully exemplified. In 

 the district described in this paper, the series, though not so complete, 

 furnishes another remarkable illustration — showing the passage from 

 granitoid rocks to others, which, though not actually volcanic, have, 

 however, approached near enough to that character to give rise to 

 aeriform explosions and the production of a rock analogous to 

 volcanic agglomerate. 



DECADE II. VOL. V.— NO. X. 29 



