ANNIVERSARY ADDRESS OP THE PRESIDENT. 87 



call larger questions has prevented me from devoting to it much of 

 the time which I can secure from daily duties. It was a question 

 to which my work in Charnwood and Xorth Wales ohliged me to 

 pay attention, and I may mention that it is now seven years since 

 I published a paper in the ' Geological Magazine ' which dealt with 

 the possible mode of formation of some spherulitic rocks in Arran. 

 This will, I hope, be my excuse for putting before you some of the 

 results of my own work, without direct reference to what others 

 may have written on the subject. The question propounded to us 

 by the study of [this class of rocks in the field is practically this : 

 Has the petrosiliceous, the spherulitic, the micrographic structure 

 been produced during the original cooling of the rock, or has it been 

 subsequently brought about ? and if it has been produced at either 

 time, is there any hope of distinguishing what I may call original 

 from secondary devitrification ? Besides studying a large number 

 of natural rocks, I have examined some artificial glasses in the hope 

 of obtaining some help from them. 



I have to acknowledge with gratitude the assistance which I 

 have received, both in information and by the gift of specimens, 

 from Professor Judd, Mr. J. A. Phillips, Messrs. Osier, Mr. F. 

 Claudet, and Mr. Frederick Siemens, of Dresden. To the last two I 

 can hardly adequately express my thanks. Each, from the funds of 

 his practical experience, gave me much information ; each made 

 me a liberal gift of specimens, Mr. Siemens even having some 

 specially prepared for me. I may add that in the time at my dis- 

 posal since these reached me, it has been impossible to do full justice 

 to the many interesting questions to which they give rise ; but I 

 hope to make them the subject of further study. 



Specimens of artificial glasses vary considerably in composition. 

 The following are analyses (furnished me by M. Claudet) of fiint- 

 g]ass (I. and II.) and a (French) bottle-glass III. * 



^ M. Pelouze also, in his article Sur le Verre, Compt. Eend. Ixiv. p. 53, gives 

 the following analyses : — 



Ordinary Glass. 



SiO^ 77-04 



CaO 7-41 



Na.O 15-51 



99-96 100-00 10000 



Alumina glasS; Magnesia glass. 



S. G. 2-380. (Easily devitrified.) 



SiO., 75-00 SiO^ 68-9 65-7 



AlA 7-60 MgO 14-9 12-0 



Na^O 17-40 CaO 73 



ISXO 16-2 15-0 



7305 

 15-16 

 11-79 



77-80 



12-50 



9-70 



100-00 



100-0 100-0 



