SoLLAS — On the Crystalline Form of Rieheckite. 



517 



Fragments of a rock similar to the so-called micro-granite of Ailsa 

 Craig are found generally, though sparingly, scattered through the 

 glacial deposits of the east coast of Ireland, from Greenore\ near 

 Carlingford, on the north, to Greystones,^ in county "Wicklow, on the 

 south. The identification of these fragments with the Scotch rock 

 depends on the presence in both of a characteristic blue hornblende, 

 known as riebeckite ; but for this there is little to distinguish them 

 from much of the granophyre of Carlingford and the Mourne Mountain 

 district, and it is by no means impossible that a riebeckite-bearing 

 granophyre may eventually be discovered as a constituent of the 

 Carlingford complex. Up to this, however, -although I have made a 

 prolonged and painstaking search, I have failed to find it. 



The fragments in the glacial deposits are usually well rounded 

 pebbles, under 6 inches in diameter, and of a remarkably compact 

 texture, which does not prevent them however from occasionally 

 containing numerous drusy cavities, which are tapestried by minute 

 ciystals of quartz, felspar, and riebeckite, the chief mineral consti- 

 tuents of the rock. In a single instance, afforded by a pebble found at 

 Portrane, the cavities are large enough to afford room for well -formed 

 crystals of riebeckite up to 5 mm. in length. Several of these were 

 successfully extricated from the rock and measured by means of a 

 goniometer with a horizontal circle, made by Fuess, and kindly lent 

 me by Professor O'Eeilly of the Eoyal College of Science. 



Our knowledge of the crystalline form of riebeckite was previously 

 very imperfect, no other faces than those of the prism and clino- 

 pinacoid had hitherto been observed. ISox is this surprising, for not 



^ Cole, Nature, vol. xlvii., p. 464. 



2 SoUas, Proc. Geol. Assoc, vol. xiii., p. 118. 



