506 Scientific Proceedings, Royal Dublin Society. 



formed prior to the schorl, as they are completely enclosed by this 

 mineral, the crystals of which are flattened on the faces of the 

 beryls. In other places the beryl is completely decomposed, and 

 has resulted in a clay of a light-green colour and unctuous feel, which 

 may be remarked in many other granites of the county Dublin, 

 but which we may thus presume to have resulted from the decom- 

 position of beryls. It will be of interest to determine whether 

 these clays contain any glucina, or whether, in the process of de- 

 composition, all the glucina is carried off in a soluble state. It 

 will also be interesting to determine whether the green earths, here 

 and there noticeable in the Dalkey granites, are really to be accepted 

 as resulting from the decomposition of beryls, or to be considered 

 as independent minerals. 



There is a further point in connexion with this occurrence of 

 beryl worth noticing. It is the evidence it furnishes as to the 

 depth and extent of the main jointing affecting the granite masses 

 of the county Dublin, and the probability that it points to some 

 of these joints developing into lodes at their contact with the mica 

 slates, which in this particular case are not far distant. 



Lastly, I look upon the existence of this well-marked jointing 

 as an evidence that Glencullen Valley is a fissure valley, which is 

 quite in keeping with its markedly rectalineal direction. 



