80 Scientific Proceedings, Royal Dublin Society. 



and attain 5 or 6 millimetres in length. The usual well- 

 marked cleavage-cracks traverse the crystals. Around these 

 grains, there is occasionally the appearance of flow-structure 

 among the smaller lath-shaped felspars. The greater part of 

 the space between the large olivine grains is occupied by large 

 plates of augite, in which the majority of the felspars, both large 

 and small, are ophitically included. The augite is of a pale 

 pinkish-brown colour, and is very fresh. The felspars are also 

 fresh and unaltered. Some few felspars attain a length of 4 or 

 5 millimetres. I have detected no glass in the sections. 



A study of the worn surface of the set is instructive. The part 

 to be ascribed to olivine as a set-mineral is at once determined. 

 The large pits are lined with broken and splintered olivine debris, 

 or often only a flooring of the mineral is left over. This mineral 

 evidently breaks up readily under attrition and impact, and, 

 although hard, must be classed with the yielding minerals. The 

 cleavage is evidently one cause of this ; probably also, in part, the 

 liability to alteration. 



We may also, from examination of this set, conclude that augite 

 cannot be regarded as a durable mineral. If it were such — at 

 least, in a degree comparable with the felspars — the set would not 

 wear so rapidly as it does. In point of fact, while the felspar con- 

 stitutes but a comparatively small part of the rock, felspar and 

 augite together constitute at least two-thirds of the rock. The 

 rapid wear and soft character of the set must therefore be ascribed 

 in part to the failure of the ophitic augite. So small a proportion 

 of felspar — actually only about some 25 per cent, of the rock — 

 unless in a quartz setting, could not be expected to resist long ; but 

 as outlasting the olivine, it confers its roughness upon the set and 

 whatever durability it can boast of. 



The qualities of this set enable me to say a word here about the 

 conditions generally influencing 



(3) Uniformity of Wear. 



The failure to comply with the condition of uniform wear 

 shown by this basalt, should not, I think, be ascribed to 

 any inequalities in the gross texture of the rock but rather 

 to its mineral composition. The conditions for uneven wear 



