68 Scientific Proceedings, Royal Dublin Society. 



posed, alike disappear from the immediate surface of the set 

 before wear has sensibly advanced on the harder minerals. Of 

 those softer minerals which do not originate as decomposition 

 products, the micas (which in the rock appear to vary in stability 

 according to their mineral surroundings in a manner not easily 

 explained) are again rapidly removed under wear. Nor does 

 muscovite, the white mica, exhibit more resistance than biotite, 

 although the latter is chemically rather unstable, while the former 

 is a very stable substance. 



The felspars are minerals which weather with some rapidity 

 under the action of atmospheric waters, although generally 

 durable when contrasted with the average life of a set. The 

 potash felspars, orthoclase and microcline, are believed to be the 

 most durable. Felspars under normal weathering sometimes 

 disintegrate or break up along cleavage directions without appear- 

 ing to suffer any sensible loss of constituents. All felspars alike 

 yield the soft kaolinite as residual product. 



There is conflicting evidence as to whether the felspars or 

 hornblendes are the more resistant against the effects of normal 

 weathering. The augites also take their place here in point of 

 stability : in basalts augite appears, however, sometimes to be the 

 last to yield. Among these minerals the felspars are the most 

 durable in the set ; the physical and not the chemical structures 

 appearing to control the durability under the conditions of 

 destruction. That the considerable degree of chemical stability 

 possessed by the felspars, hornblendes, and augites is of first 

 importance in the value of many sets is, however, evident. 



Quartz, garnet, tourmaline, magnetite, and epidote, are,, 

 chemically, very durable minerals; the first -named practically 

 unaffected by conditions of weathering and by most solvents. 

 Pyrites is not a very stable mineral, being liable to oxidation. 



Olivine is rapidly decomposed and softened by normal atmo- 

 spheric influences. In basalts it is invariably the first to go. It 

 becomes iridescent, and breaks up along irregular cracks. In 

 the street the severe conditions of solvent denudation very 

 probably act to hasten its destruction. 



In the case of some igneous rocks, of somewhat open structure,. 

 e.g., certain granites in which mineral alterations have taken place, 

 the surface-waters of the street may visibly penetrate and discolour- 



