GRANITE 733 



south from Dublin, through the counties of Wicklow and Carlow into Kilkenny ; it 

 occurs on the south-eastern coast of Down, and around Newry ; the range of the 

 Mourne mountains is granite, which again appears in small and isolated protrusions 

 in Derry and Tyrone, and in Cavan. In the western portion of Donegal there is a 

 large extent of this rock, which here partakes of a gneissose character ; and again, in 

 the west of Galway, granite covers a considerable area. The granite of the Wicklow 

 range is the most extensively used. It varies in its quality, that near Kingston being 

 coarse and hard, while that from Ballyknocken, or Golden Hill, is much finer, and 

 therefore fitted for ornamental work. The granite of Down is of a darker colour and 

 finely crystallised. It is extensively quarried at Newry, and sent by water to the 

 north of Ireland. 



The Galway granite is of a reddish colour, containing large crystals of flesh-red 

 felspar. That of Mayo is of a dark bluish grey colour, while that of Tyrone is 

 reddish. 



The Irish granite averages 170 Ibs. per cubic foot, its extreme weights being 143 Ibs. 

 and 176 Ibs. After 88 hours' immersion in water, it was found that a cubic foot of 

 the granite of Newry and Kingston absorbed about lb., that of Carlow nearly 2 Ibs., 

 and the granite of Donegal 4 Ibs. of that fluid. These facts are important in con- 

 nection with the use of these rocks for building purposes. Wilkinson's Practical 

 Geology and Ancient Architecture of Ireland. Sir R. Kane's Industrial Resources of 

 Ireland. HulVs Building and Ornamental Stones. 



Granite is worked to a small extent at Shap Fell in Westmoreland, and at Mount 

 Sorrell in Leicestershire. The rocks worked as and called the Grooby granite may 

 perhaps more properly be termed Syenites, in some cases assuming the character of a 

 syenitic granite, in others of a syenitic greenstone. These are worked extensively for 

 4 pitching' and for macadamising roads. 



Until recently it was an opinion almost universal among geologists that granite 

 had been formed by igneous fusion, and had solidified from a molten condition, under 

 enormous- pressure ; the theory being supported partly by the crystalline and un- 

 stratified character of the rock, but chiefly by the fact that almost every large mass 

 of granite gives off numerous ramifications or veins which penetrate the surrounding 

 rocks, and which appear to have been formed by injection of the molten granite into 

 neighbouring fissures. Evidence has, however, accumulated during the last few years 

 tending to modify this theory very materially. Thus, it has been found that the 

 specific gravity of quartz which has undergone fusion is never higher than 2'3, whilst 

 that of the quartz in granite reaches 2*6 ; but it should be remembered that the 

 pressure under which granite has evidently been formed must, as Dr. Haughton has 

 pointed out, tend to increase the density of the silica. Again, many minerals occur- 

 ring in granite, either as essential or as accessory constituents, are such as could not 

 have existed at a high temperature ; some of them, indeed, containing a notable pro- 

 portion of water. Many of these minerals, moreover, are so highly basic that it is 

 difficult to understand how they could have been fused in the presence of free quartz 

 without entering into chemical combination with the silica. Finally, the constituent 

 minerals of granite have, in most cases, not solidified in the order of their relative 

 infusibility ; the quart/, which is the most infusible, having evidently solidified last, 

 since it commonly bears the impress of the felspar crystals. 



Much light has also been thrown upon the probable origin of granite by the 

 researches of Mr. H. C. Sorby on the microscopical structure of crystals : a subject 

 which has been followed up by Dr. Zirkel and other observers. The crystals of many 

 minerals contain cavities partially filled with liquid, which has evidently been caught 

 up and mechanically enclosed during the formation of the crystal : a phenomenon 

 frequently witnessed in the crystallisation of artificial salts in the laboratory. From 

 the nature of their formation, it is evident that such cavities must be entirely filled with 

 liquid at the temperature at which they have been formed ; any vacuities that after- 

 wards appear being due to subsequent contraction, resulting usually from a reduction 

 of temperature. If the crystal, therefore, be exposed to heat until the liquid, by its 

 expansion, entirely fills the cavity, the temperature at which the crystal was originally 

 formed will be at once indicated. Such results will, however, require considerable 

 modification if the mineral should have been formed under the effects of great pressure, 

 as was certainly the case with granite. As crystals deposited from aqueous solution 

 contain fluid-c&vities, so those obtained by sublimation enclose bubbles of vapour, 

 giving rise to gas- or vapour-cavities ; whilst substances solidifying from fusion, such 

 as glass or slag, contain portions of solid matter, in what are termed glass- or 

 stone-CAvitieB, according to the nature of the enclosed solid. Now, when the con- 

 stituent minerals of granite are subjected to microscopical examination, they exhibit 

 numerous cavities, of which some contain liquids, others enclose solids, and a few 

 merely vapour. These cavities are most abundant in the quartz, some varieties of 



