Hull — On the Different Varieties of Irish Paving Setts. 95 



that should the stone be wanting in homogeneousness it would be 

 liable to break down under traffic, the softer portions giving way 

 before the harder, and thus causing the blocks to collapse. It will 

 be found that all the rocks used for the production of setts possess 

 this quality, though differing from each other in other respects. 



(b) Toughness. — I prefer the term to hardness, inasmuch as 

 many very hard rocks — such as flint, chert, and quartzite, are 

 deficient in toughness ; and are, consequently, liable to crack, and 

 splinter upon percussion. Such rocks are therefore unfitted as 

 materials for paving-setts, which ought to be capable of with- 

 standing the percussion caused by the sudden shock of the wheels 

 of heavily-laden vehicles passing over their surfaces, not to speak 

 of those caused by the iron-shod feet of dray-horses. 



(c) Roughness of Surface. — This is a quality not less valuable 

 than that of toughness, and the best varieties of paving-setts are 

 those which combine these three qualifications. It has been found 

 by experience that some of the harder kind of paving-setts are 

 liable, after some wear and tear, to have the surfaces worn smooth, 

 and actually to become polished. In this state they become dan- 

 gerous for street traffic ; and, notwithstanding their durability, 

 they are held in less favour in the large manufacturing towns of 

 the North of England than was formerly the case; and other 

 kinds of stone, though somewhat softer and less durable, are 

 preferred, in consequence of their ability to maintain a rough 

 surface. 



II. Varieties of Stones for Paving-setts. — I now pass on to con- 

 sider briefly the varieties of stone suitable for the manufacture of 

 paving-setts, and therefore combining in a greater or less degree 

 the qualities previously enumerated. They may be considered 

 under three heads: — 



(a) Those of sedimentary origin, such as grits and sand- 



stones. 



(b) Those of igneous origin, of a granitoid character; 



including quartz-porphyries. 



(c) Those also of igneous origin, but belonging to the 



variety commonly known as " whinstone " ; in- 

 cluding basalt, dolerite, diorite, and felstone. 



