72 Scientific Proceedings, Royal Dublin Society. 



An opaque black ore — probably magnetite — is present in scattered 

 crystals. The rest of the field is occupied by felspars and quartz, 

 the latter being quite subordinate to the former. These hard 

 constituents compose some 70 per cent, of the rock. As it 

 requires polarized light to differentiate these, they do not defi- 

 nitely appear in the figure. The felspars are columnar and lath- 

 shaped, making up an almost continuous field of crystals orientated 

 in every direction between the coloured constituents. In some 

 cases, a felspar crystal may attain the length of as much as 4 or 

 5 millimetres ; and in this case the rhombic pyroxene and augite 

 will be included within it. More generally the felspars are of 

 about the same dimensions as the coloured constituents. The 

 smaller felspars are almost certainly plagioclase. Some of the 

 larger may be orthoclase. The felspars are fresh, and unclouded 

 in the sets examined. The quartz, which is fairly abundant, is 

 interstitial, and also in part pegmatitic. 1 



The chief feature of the rock, from the present point of view, 

 is the uniform distribution and fine state of subdivision of the 

 various constituents. 



When we examine the surface of the worn set, we see that the 

 abundant felspars and intergrown quartz undoubtedly support 

 the wear. It is difficult to determine how far the enstatite and 

 augite assist ; but it can be seen with certainty, in many cases,. 

 that the coloured constituents lie in the hollows. In fact, we may 

 conclude, with little risk, that the cause at once of the durability 

 and slipperiness of the set is apparent in its fine texture and the 

 too uniform distribution of the soft minerals of the rock. The 

 roughnesses in a set from such a rock can only be on the 

 most minute scale. The surface is, in point of fact, matt. A 



1 The quartz may be at once detected by its greater birefringence when the method 

 of double transmission is used in examining tbe section. Examination of a slice of tbe 

 usual thickness in the ordinary way (the polarized ray being transmitted from beneath 

 the section, and so only traversing it once) differentiates between quartz and felspar 

 only in so far that the quartz appears in a whiter grey. Using a reflector close 

 beneath the slide, and a polarized ray directed vertically downwards, the retardation- 

 due to the doubled thickness is sufficient to raise the colour of the quartz to straw- 

 yellow, and leave tbe felspar for the greater part still in the greys and grey-white. 

 (See Proc. R. D. S., "On an Improved Method of Identifying Crystals in Rook- 

 Sections," vol. ix., p. 485, and vol. x., p. 1.) Mr. Teall, in his description of the rock, 

 remarks upon the use of birefringence in distinguishing between the quartz and felspar. 



