Hyland — On some Spherulitic Rocks from Co. Down. 433 



address for particulars of the crystalline aggregates and spheru- 

 litic bodies formed in artificial glasses by the action of heat short 

 of fusion. 



Before concluding this portion of the Paper, it seems well to 

 recall once more the experiments of Daubree 1 on common glass. 

 These were performed in closed tubes in the presence of water. 

 They are remarkable as showing the powerful solvent action which 

 water at high temperatures, and under considerable pressure, may 

 have upon many substances which are not affected by it under ordi- 

 nary temperatures and pressure. He found, that a glass represent- 

 ing a homogeneous silicate of soda and lime was converted into 

 crystallized quartz and pyroxene with spherulitic growths at a com- 

 paratively low temperature. The process which has taken place in 

 nature must, according to Professor Bonney, have been analogous 

 to this experiment — that is to say, when " devitrification," in the 

 strict sense of the word, has been produced in a rock once glassy 

 " the agents of change have been pressure, water, heat — the 

 elevation of temperatures being probably, in most cases, very 

 moderate." 2 



A brief reference will now be made to the question of the age 

 of the outbursts of acid material in the Mourne Mountains. In a 

 paper read before the British Association, Messrs. Hull and Traill 

 recorded their opinion that the granite of Mourne is of Mesozoic 

 Age. 3 On the other hand, the late Professor Harkness suggested 

 that it was irrupted during the interval between the Carboniferous 

 and Permian period. All that can be confidently affirmed from 

 the available evidence is, that the Mourne Granite is Post-Carbo- 

 niferous on the one hand, and older than the Miocene Period on 

 the other. 4 To the latter period of volcanic activity are referred 

 the numerous basaltic dykes by which it is penetrated. On 

 the other hand, the basaltic dykes, which terminate at the margin 

 of the granite, are more ancient than it. The order of succession 

 has therefore been in the Mourne Mountains : — 1, basic materials \ 

 2, acid ; and 3, again basic. 



1 Etudes Synthetiques de Geologie Experirnentale, tome i., pp. 159—171. 



2 I. c, p. 93. 



3 Brit. Ass. Rep., 1871 ; Trans, of Sections, p. 102. 



4 Prof. E. Hull, in Memoir to Sheets 60, &c, p. 29. 



