12 Mr. J. A. Phillips on certain Slates, Felsites, and Elvanites 



than about one-sixth of a second. Much about the functions of 

 the eye is without doubt known ; but I strongly insist that 

 further inquiry should take place before it be assumed that " the 

 known properties of light afford a complete elucidation of the 

 whole mechanism of vision and the use of every part of the visual 

 apparatus/' 



III. Note on certain Slates, Felsites, and Elvanites occurring 

 near Knockmahon in the County of Water ford. By J. Arthur 

 Phillips*. 



THESE rocks are exposed in the sea-cliff east of the vil- 

 lage of Knockmahon, and are much contorted, their 

 general strike being from N.E. to S.W. The dykes or bands 

 of felsite and elvanite often appear to be interstratined with 

 the Lower Silurian fossiliferous slates of the district, and to fol- 

 low the line of their bedding without producing any marked 

 change in the character of the rocks in immediate contact with 

 them. They can in many instances be traced for considerable 

 distances, but appear to be frequently lenticular in shape, thin- 

 ning out horizontally at either end, whilst the workings in the adja- 

 cent mines have afforded examples of a similar dying out in depth. 



At the extreme south-west corner of the townland of Tankards- 

 town is a well-detined north and south fault, immediately to the 

 west of which the rocks are traversed from N.W. to S.E. by the 

 productive copper-bearing lodes of Knockmahon. 



Four specimens of the more characteristic rocks from the 

 above locality were selected for microscopical and chemical 

 examination, and afforded the following results. 



Elvanite, spec. grav. = 266. — A section was prepared from a 

 band of compact elvanite of a bluish-grey colour, which, when 

 freshly broken, shows imbedded crystals of quartz and felspar in 

 an amorphous matrix. This band, which is of considerable but 

 varying width, is in the immediate vicinity of the Tankardstown 

 lode, and has been intersected by the workings of the mine at a 

 considerable distance from the cliff. 



Examined under a ^-inch objective, this rock is seen to be 

 composed of an amorphous greyish matrix in which are por- 

 phyritically imbedded crystals of quartz and felspar, the latter 

 being chiefly oligoclase. In addition to these, a few small 

 crystals of some hornblendic mineral are sparingly disseminated 

 throughout the mass. It was further observed that the larger 

 quartz crystals are sometimes penetrated by crystals both of 

 felspar and hornblende; and when examined under a high powe^ 

 the quartz is seen to contain fluid-cavities. 



* Communicated by the Author. 



