96 Scientific Proceedings, Royal Dublin Society. 



(a) Grits and Sandstones. — The formation which yields this 

 class of paving-setts in greatest quantity is the carboniferous; and 

 beds belonging to the millstone grit division in Lancashire and 

 Yorkshire are largely worked for paving-setts. This rock, owing 

 to its granular structure, is probably the softest of all the varieties 

 of stone capable of being used for paving purposes ; still, it is very 

 largely used in the streets and roads of the North of England for 

 pavements ; and is found, when properly selected, to answer well 

 where the traffic is not excessively heavy. Gritstones have .the 

 useful quality of preserving a rough surface ; and can be set quite 

 close, side by side. As far as I am aware, there are no paving- 

 setts made from the carboniferous rocks of Ireland, though I have 

 no doubt some of the beds of grit of this formation in the counties 

 of Sligo, Fermanagh, Leitrim, Donegal, &c, are capable of pro- 

 ducing them. In Belgium the gritstones of the Upper Devonian 

 formation, known as the " Psammite du Condroz," are very 

 largely used for paving the streets of the manufacturing and 

 other towns. 



(b) Granitoid Varieties. — This group includes not only granites, 

 but quartz-felstones and porphyries ; in which the constituents are 

 quartz, felspar, with mica or hornblende as accessories. These 

 components are, more or less, in a crystalline condition, and have 

 solidified from a state of igneous fusion. In consequence of this, 

 the mineral constituents are firmly bound one to the other, and a 

 condition of "toughness" is imparted to the mass favourable to 

 its use for paving-stones. The presenoe of mica, if in large flakes, 

 would prove a source of weakness, in consequence of its want of 

 cohesion with the other minerals ; but when in minute flakes, this 

 mineral, by its disintegration, enables the stone to preserve a con- 

 stantly rough surface. 



Paving- setts belonging to this group are worked at Bessbrook, 

 G-oragh Wood, and Castlewellan, in Ireland ; and at Mount 

 Sorrell, in Leicestershire. 



(c) Whinstones. — The stones belonging to this third division 

 differ from those already described in texture and composition. 

 They consist of crystalline aggregates of felspar and augite, or 

 felspar and hornblende, together with magnetic iron-ore dis- 

 seminated in minute grains, and with occasionally other minerals, 

 such as olivine, and chlorite, in small quantities. The presence of 



