Joly — The Petrological Examination of Paving-Sets. 79 



amount of alteration is hardly enough to justify the latter 

 term. Some specimens which I took from the quarries some 

 years ago, have much the same petrographical characters as the 

 set, but alteration has gone much further. 1 



The durable material is the sufficiently abundant felspar. 

 The softer materials are the alteration-products and — consider- 

 ably harder — the augite. It is the balance of these which 

 confers upon the set its qualities. The worn surface, in fact, 

 shows that the summits are the felspars ; while the hollows are 

 occupied by coloured substances and the iron ores. There are no 

 extended and smooth table-lands. The surface is markedly rough 

 to eye and to touch. The felspar-areas are not extensive enough 

 to offer polished or smooth eminences of any injurious extent. 

 They are well broken up by the aggregated grains of the softer 

 minerals. These aggregates represent depressions on the surface 

 of the set, and may measure up to 3 or 4 millimetres in diameter. 

 The connexion between the worn surface-characters of the stone 

 and its microscopic constitution is evident. That a rock with 

 sucli a proportion of soft material would be fairly, not very, dur- 

 able is to be expected; and that a hard material, intermixed in such 

 proportions and with such distribution through a soft material, 

 must afford a rough surface under attrition, might have been 

 safely predicted. 



The Ballintoy Olivine Dolerite. 



The last set to be described here is one of typically soft 

 character. The set is from the basic rock of Ballintoy, County 

 Antrim. It never gets slippery, but wears rather rapidly and 

 unevenly. This is, then, the first case we have met of a set 

 failing to fulfil the condition of wearing uniformly. The rock 

 is heavy and black, with rough fracture. It must be described 

 as an ophitic olivine dolerite. Specific gravity 2982. 



The olivine occurs in large crystals (see the photographed 

 section, magnification twelve diameters, fig. 2, Plate V.) of 

 perfectly fresh character, as well as in small scattered grains. 

 Some of the larger grains would not have fitted into the figure, 



1 Mr. Watts (" Catalogue of Irish Minerals ") describes the Arklow rock as consisting 

 of ophitic dolerites or diabase?. 



H2 



