170 Archdeacon Verschoyle on the North Coast of Mayo and Sligo . 



augite. At this side the character of the bed differs shghtly from that of 

 RosSj being more varied in structure, and might be applied to ornamental pur- 

 poses, if not too hard to be wrought satisfactorily. The felspar, which occurs 

 abundantly at Ross, presents unusual varieties in the crystallization. The 

 crystals appear to be rectangular prisms, having the terminal angles replaced 

 by triangular planes ; but on close inspection they are found to consist of 

 two or four prisms attached laterally. Striae may be seen with a magnifier on 

 the planes parallel to the edges of that replacing the angle of the prism ; and 

 as these striae diverge from the line of junction, the crystal has a feathered 

 appearance, as represented in the wood-cuts. The 

 conditions under which the complicated arrangement 

 of the ultimate particles, and of the simple and com- 

 pound crystals were effected, apparently indicate per- 

 fect tranquillity, complete fusion, and very slow cool- 

 ing of the mass. This conjecture is supported by the 

 fact, that the most perfect crystals are found about the centre of the bed, the 

 part which would longest retain its heat and fluidity. 



The occurrence of trachyte (a formation hitherto unobserved in the British 

 Islands) at the shores of the bay of Killalla, is an interesting discovery. Of 

 the period when the rock was formed, it can only be said with certainty, 

 that it is newer than the mountain limestone, and older than the system of 

 trap dykes, as it lies on the former, and is penetrated by No. 5, one of the 

 latter. 



