448 Joseph Nolan — Volcanic District of Slieve Gullion. 



to produce aeriform explosions and the shattering and trituration 

 of the superincumbent rocks, they do not appear to have been 

 powerful enough to raise the igneous mass sufficiently near the 

 surface to produce lava or scorise. This was probably due to the 

 operation of two causes, one of which I have already referred to — the 

 sudden consolidation of the crystalline magma on losing its inter- 

 stitial fluids — the other, and perhaps principal cause, the immense 

 weight and volume of the displaced materials. To this latter cause 

 Mr. Scrope refers the formation of fragmental rocks in the Upper 

 Eifel, very similar to those just described. These, he informs us, are 

 " principally, and in some- instances almost entirely, composed of 

 broken greywacke slate and sandstone, more or less affected by heat, 

 and pulverized. . . . There is an appearance as if the volcanic energy 

 had been damped and impeded by the mass of transition and 

 secondary strata which it had to pierce, and perhaps by the fragile 

 nature of the greywacke slate, which, shattered and pulverized by 

 the first few aeriform explosions of every eruption, would be likely 

 to accumulate in great volume above and within the vent, and stifle 

 its further activity." — {Volcanos, p. 376, et seq.) 



Besides the felspathic rock mainly associated with the agglomerate, 

 there are others of a basic character, chiefly dolerites and melaphyres. 

 These usually occur as bosses and protrusions through the fel- 

 spathic rocks, and are generally tuffose at the surface, graduating 

 imperceptibly into the agglomerate. One of the most remarkable of 

 them is to be seen at Glendooey, west of Carrickbroad, where it rises 

 as an abrupt dark mass on the side of the hill, and, crowned with an 

 ornamental turret, forms a very pretty and conspicuous object. It 

 has all the appearance of a volcanic vent, and the tract of dark basic 

 rock, which spreads to the southward, probably proceeded from it. 

 This latter has been microscopically examined by Professor Hull, 

 who finds it to be composed of triclinic felspar and epidote, the 

 latter, however, occurring as a secondary mineral, and not improbably 

 having replaced augite. (See Geol. Survey Exi:)lanation to accom- 

 pany Sheet 70, p. 19.) 



Although at first it might appear that these basic rocks are newer 

 than the felstone porphyry, yet thei'e is good reason for believing 

 that they were formed about the same time, that is, during the con- 

 tinuance of the same volcanic action. Their graduating into the 

 agglomerate is a very good reason why we should consider them 

 coeA'al with it, and their protrusion through the porphyry is probably 

 more due to the great differences in the respective fusibilities of 

 these rocks than to later eruptions. Thus, Prof. Jukes writes : — 

 *' Granite might become solid at a temperature that would keep 

 felstone and trachyte still fluid, and these might solidify at tempera- 

 tures which would keep molten all greenstones, basalts, and dolerites, 

 so that from the very same stream of igneous matter proceeding 

 from the interior to the surface of the earth the more readily fusible 

 portions might be successively squeezed out, as it were, as the infusible 

 ones solidified, and contracted in consequence of that solidification. 

 This action might take place in spite of the greater specific gravity of 



