78 Scientific Proceedings, Royal Dublin Society. 



comparable in dimensions with the grains of quartz and felspar, 

 provide the coarser and deeper inequalities of the worn surface. 

 The conditions here prevailing are, as experience shows, those 

 affording a set at once sufficiently durable, maintaining its surface- 

 roughness, and wearing uniformly. 



The Arkhw Dolerite. 



The next set displays a very different surface to any of the 

 foregoing. It is described as follows : — " Never wears smooth, but 

 is rather soft." The rock is that of the Parnell quarries, south of 

 Arklow. It is a dark, heavy rock, green in colour, and breaking 

 with a rough fracture, and to the unaided eye reveals its grain ; 

 although the uniformity of colour renders this less conspicuous 

 than would be the case in granitic or syenitic rocks of similar 

 coarseness of structure. Specific gravity, 2"846. 



The microscope (see fig. 1, Plate V., magnification, twelve 

 diameters) shows that the rock, which is a dolerite or diabase, 

 is fairly rich in alteration-products. These are of two kinds : 

 sometimes evidently a chlorite, pale green, plumose, with 

 steely-blue extinction. These areas extend into the augites 

 present in such a way as to suggest that they represent 

 materials derived from this mineral. The other decomposi- 

 tion-product is flaky and nondescript, only showing a 

 partially active or spangly field between crossed nicols. This 

 appears to be derived from the felspar, and is probably in part 

 calcific. Both these substances must be classed as soft material. 

 The fresh and abundant augite present must also be classed as 

 badly-resisting material. The relations of this augite with the 

 felspars is variable. Generally the augite is allotriomorphic 

 but is also found idiomorphic. In one or two places it is 

 ophitic. Again, in places, the felspar is moulded on shapeless 

 augite outlines. Magnetite is fairly abundant. The only 

 important hard material present is the felspar. It constitutes 

 about 77 per cent, of the rock. It is in crystals of elongated 

 habit, which vary from very small to 3 or 5 millimetres in 

 length, and is, for the greater part at least, plagioclase. It 

 is occasionally clouded, but retains activity in polarized light 

 The rock must be described as a dolerite or diabase. The 



