ANNIVERSARY ADDRESS OP THE PRESIDENT. 95 



dritic structure *, are common ; and not seldom tiny spherulites occur, 

 whether singly or in crowded groups, of that indefinite external 

 character already mentioned. In short we have a number of struc- 

 tures similar, so far as I can judge, to those figured by MM. Fouque 

 and Levy in their magnificent work ' Mineralogie micrographique,' 

 plates xi., xii. (2), xiv., xv., and xvi. (1): in the last, I believe, 

 at any rate the larger spherulites are of anterior consolidation. I have 

 found characters more or less similar prevail in the " felstones " 

 beneath the base of the Cambrian of North and South Wales, in the 

 compact lavas from Charnwood, in the more compact of the Ordovician 

 lavas from "Wales, and in many other similar rocks. It is, in fact, 

 generally found in those " felstones " which have a compact, smooth, 

 and sometimes subconchoidal fracture, but hardly so much as a 

 glimmering lustre. 



Hence it appears to me that the petrosiliceous structure of the 

 above-described character is probably always the result of secondary 

 devitrification, in which pressure and water (acting for a long time) 

 have been more important than heat. It has nearer relations to the 

 microgranulitic structures found in certain vein-granites and intru- 

 sive felstones than to the trachytoidal structure of lava-flows ; which 

 is not surprising, seeing that the former structures have probably 

 been set up under considerable pressure and in the presence of water. 

 Thus we appear to have two groups of structure : one, the trachytoidal 

 and ophitic; which are more generally the results of drier and less 

 constrained cooling ; the other, the petrosiliceous, microgranulitic, and 

 granitoidal, indicative of the presence of some water and the existence 

 of much constraint, the first of these three being probably almost 

 entirely a structure of secondary origin ; for I expect that we shall find 

 on further study that we shall be able to distinguish even the more 

 minutely microgranulitia rocks from the truly petrosiliceous ; but on 

 this point I will not venture to speak at aU positively, as I have not 

 been able to study so many specimens of these vein-granites as I 

 should wish to have done. Still I think we may safely affirm that 

 the majority of the petrosiliceous rocks owe their structure to a 

 peculiar form of subsequent devitrification, and so, as altered rhjolites, 

 obsidians, and pitchstones, belong more properly to the metamorphic 

 rocks (of igneous origin). 



I have throughout spoken, as I stated I should do, with little 

 reference to the work of others, because I thought that there might 

 be a certain interest and advantage in presenting what I had to say 

 from a personal point of view, since on nearly every point I have 

 striven to form an independent conclusion, and often the result of 

 many hours' work has been condensed into a few words. It has 

 been work, I fear you will say, leading to little result ; but perhaps 

 its very incompleteness may suggest lines of research to other workers. 

 Another reason why I have referred little to the investigations of 

 others is, that in each case one ought in justice to be sure of namino- 

 the original observer. Now to do this would have involved much 



* I use the last term for want of a better name to express cases where the 

 minerals resemble crowded branches or rootlets. 



