Joly — The Penological Examination of Paving- Sets. 67 



agencies. Thus, while among the sedimentai-y rocks, the most 

 perfect examples of cleavage structure are found, certain igneous 

 rocks, e.g. some basalts, exhibit this structure, apparently brought 

 about by dynamical action. 1 Joint planes are generally avoidable 

 in the selection of the stone, being only locally distributed in the 

 rock, and not affecting the minute rock-structure. 



The Chemical Stability under the conditions of wear can, in 

 the absence of direct experiment, only be approximately inferred 

 from our knowledge of the weathering properties of the minerals 

 Tinder normal conditions. Daubree has shown (Joe. cit.) that 

 attrition assists the activity of solvent and chemical actions, as 

 indeed might have been anticipated, if only from its effects in 

 removing the residual materials, and thereby eliminating their 

 protective action. 



In experiments in which I compared the solvent action of 

 fresh water and sea water on some rocks and minerals, I found 

 that orthoclase is decomposed at least fourteen times as fast in 

 sea water as in fresh, and that various silicates of basic rocks 

 also showed a feebler resistance in sea water. Daubree, using 

 only chloride of sodium in the solvent, found that there was 

 actually a protective effect exerted by this solvent, when the 

 behaviour of orthoclase was compared in the salt solution and in 

 distilled water ; attrition being applied in both cases. Neither 

 experiments are quite applicable to the case of the paving- set, 

 and, pending direct experiment, a certain conclusion as regards 

 the weathering resistance cannot be arrived at. 



The mechanical actions are, however, so intense, that our 

 powers of prediction, as regards the qualities of a set, are in nearly 

 all cases fortunately independent of the precise relative chemical 

 stabilities of the constituent minerals. In the first place we do 

 not expect that the stability of the softer minerals would enter 

 the question ; their removal, when in any considerable aggregates, 

 is sure to take place from the immediate surface before any 

 chemical instability can manifest itself. Thus the soft minerals 

 kaolinite, serpentine, and talc, which are very resistant chemically ; 

 and calcite, the zeolites, and the chlorites, which are easily decom- 



1 Some basalts occurring at "West Loch Tarbert, Jura, show such a cleavage struc- 

 ture in wonderful perfection.' 



