Joly — On the Minerals of the Dublin and Wickloiv Granite. 53 



Externally they show well and sharply-developed faces, both 

 prismatic and basal, and the angles of the hexagonal prism. But 

 here the resemblance to beryl ends ; they are neither transparent 

 nor translucent. The vitreous surface and homogeneous appear- 

 ance of beryl are wanting. They are opaque, dull, rough, and 

 piebald : some dull green and white, some dull green and dull 

 rusty brown. They are in fact a different mineral from beryl in 

 all but external form. They are found up to about half a kilo in 

 weight. Sometimes the cores of the hexagons are eaten out into a 

 cavernous tube lined with rusty matter. Occasional cracks crossing 

 the prism recall the imperfect basal cleavage of beryl. 



Their specific gravity shows at once that they are not, or only 

 in part, composed of beryl. I found it to be 2-620 taken on a 

 specimen of average appearance, free from hollows, weighing fifty 

 grammes. The lowest specific gravity recorded by Dana is 2*63 ; 

 by Des Cloizeau 2*67. I have mentioned that the specific gravity 

 of a specimen of normal beryl from Glencullen was found to be 

 2-722. 



The specific heat is hardly abnormal so far as my experiments 

 on beryl go. Three experiments were made on the same specimen 

 used in ascertaining specific gravity : — 



(a) 0-21554. - 



(b) 0-21446. 



(c) 0-21691. 



Mean specific heat = 0-21563. 



On breaking up the crystals they are found to present inter- 

 nally the same appearance as regards colour and lustre as exter- 

 nally. 



I have seen no complete hexagons of this altered beryl. This 

 is noteworthy. One side or one end of the hexagon invariably 

 passes insensibly into the orthoclastic matrix, that again insensibly 

 passing into granitic mixture with quartz and mica. Tourmaline 

 abuts against the faces in many cases, but is easily peeled off, 

 leaving a clean, smooth surface beneath. It does not penetrate or 

 grow into the prism. With the orthoclase it is different. It is in 

 that case impossible to say, on a fractured surface, where ortho- 

 clase begins or prism ends. Nor are these crystals ever found 



