Joly — The Penological Examination of Paving-Sets. 65 



looks chalky. When so far gone, only the assurance of a practical 

 test of the set should induce us to sanction its use. 



The granitoid rocks of intermediate composition, the syenites 

 and many diorites, are, in general, valuable rocks for street- 

 pavement. The typical syenite is a rare rock. The diorites are 

 fairly abundant. As in the case of the granites, mere cloudiness 

 of the felspar of the intermediate and basic granitoid rocks is not 

 sufficient evidence upon which to reject the rock. But these rocks 

 possess no quartz ground-mass generally, and, relying mainly 

 on the felspars for their hardness and durability, are more 

 dependent on the freshness of this constituent. It may be stated 

 that a greater degree of freshness is required in the felspars of the 

 more basic granitoid rocks than in the acid granitoid series. The 

 coloured constituents, more abundant in the former series than 

 in the latter, do not in either case appear to play a leading 

 part in sustaining the wear to which the set is exposed. 



A familiar variety of rock-structure is the porphyritic. In this, 

 the growth of the rock has not been continuous, but the conditions 

 of development of the minerals have altered during their forma- 

 tion, so that a larger and older generation exists in a ground- 

 mass of smaller and younger individuals; or, it maybe, in a ground- 

 mass not completely crystallized, and therefore partly in the form 

 of glass. The availability of such rocks as paving-sets depends on 

 the extent to which the porphyritic structure is developed and on 

 the state of the ground-mass. If the large crystals are very large — 

 say, one centimetre (one-half inch) and upwards in greatest dimen- 

 sions — the risk of rapid disintegration by loosening of these, or by 

 their break-up, must be borne in mind. Or, on the other hand, if 

 resistant and hard, a slippery surface of polished and rounded 

 prominences may render the set dangerous. The last will be the 

 greatest risk in the case of porphyritic orthoclase, as in many 

 granites ; the former in the case of large crystals of mica, olivine, 

 augite, etc. In the absence of experiment, such rocks are not to 

 be recommended. 



The presence of glass in the ground-mass should certainly be 

 regarded as a danger. It is soft, brittle, and liable to devitrifica- 

 tion. If composing any considerable part of the ground-mass, the 

 rock should be rejected. Actual rough glassy lavas have indeed 

 been used for paving where other stone was not procurable ; but it 



