Joly — The Petrological Examination of Paving- Sets. 73 



more concentrated distribution of the soft minerals would have 

 given a better set. As regards durability, the set will be, on the 

 other hand, for these very reasons, exceptionally good. The smooth 

 wear of this set leads to another evil on the score of slipperiness : 

 the smooth rounding of the edges. In the case of the particular 

 set here referred to, the upper surface is rounded off where it 

 meets the vertical faces in the most hopeless fashion. 



The White Caernarvon Granite. 



A few sets have now to be examined labelled " White granite, 

 Caernarvon." These are further described as " Slippery." They 

 are in a better condition, however, than the enstatite diorite set 

 already described. 



This granite is compact and fine-grained ; of a bluish- white or 

 grey colour ; sparsely mottled with specks of a dull greenish 

 mineral which under the lens appears micaceous in structure. It 

 is heavy for a granite : specific gravity, 2"702. 



The rock composing these sets is possessed of many of 

 the features common to the granites of Wales. It is almost 

 without mica; this being apparently represented by a chloritic 

 mineral having a very feeble dichroism, its pale greenish colour 

 darkening a little to vibrations along the traces of cleavage. 

 Between crossed nicols, the mineral remains so dark that only in 

 thin specimens or with very strong light can the deep prussian- 

 blue of its interference-colour be seen. It is probably one of the 

 pennines. Certain radiated or fibrous and colourless constituents, 

 ■often intimately associated with the chloritic mineral, assume, with 

 crossed nicols, the bright colours of potash mica. The rest of the 

 rock may be said to be composed of the hard minerals felspar 

 and quartz. The quartz is abundant ; interstitial most generally ; 

 but very exceptionally it appears with a true crystallographic 

 face. (See Teall's " British Petrography," pp. 318 et seq.) 



The felspars are mostly quite irregular in form, and very 

 often crowded with an alteration product of elongated prismatic 

 habit, polarizing in brilliant colours. Again, it is sometimes 

 quite fresh and transparent. Its optical activity, when rotated 

 between crossed nicols, is found to be always preserved. Rarely 

 is the felspar idiomorphic, and still more rarely does it reveal 



