76 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



dolerites. [N'otwitlistandmg what has been written of late, I can- 

 not admit that some of the altered peridotites and the serpentines of 

 the Apennines are other than intrusive, if not in Eocene, at any rate 

 in the latest Cretaceous strata, and thus, as deep-seated intrusions, 

 cannot be in any case older than the earliest part of the Tertiary. 

 Yet these are undistinguishable in all essential characters from 

 olivine rocks and serpentine, which would generally be regarded as 

 Palaeozoic or, in some cases, Mesozoic. At our last meeting Professor 

 Judd told us of Tertiary peridotites, picrites, and gabbros in the He- 

 brides not to be distinguished from similar rocks of far earlier dates. 

 Indeed some authors have been so impressed with their ancient 

 aspect as to insist on classing these gabbros with the norites of the 

 Upper Laurentian. 



We find yet stronger instances of similarity among the andesites 

 and rhyolites. Mr. Teall*, in his excellent papers on the Cheviot 

 rocks, has shown that, except for alterations which can only be 

 attributed to the effect of time, some of the porphyrites of that region 

 are chemically and mineralogically undistinguishable from the hyper- 

 stheniferous andesites of Teitiary or yet more recent age. Mr. S. 

 AUport has shown that the devitrified perlitic rock of the Wrekin, 

 which is indubitably older than the Lingula Flags, and in all proba- 

 bility is one of the later Pre-Cambrian lavas, is as nearly as possible 

 identical, chemically, mineralogically, and structurally (except for 

 devitrification) with the perlitic obsidian of HLinik, near Schemnitz, 

 and corresponds very closely with another Post-Secondary obsidian 

 from Hungary. The red felsite, which in North Wales is found 

 below the base of the Cambrian series, exhibits in some localities a 

 fluidal structure, and every indication of having once been a true 

 glass, and is chemically identical with the above rock from Schemnitz; 

 while several of the lavas of the Ordovician series in Wales, as has 

 been pointed out by Mr. Eutley and myself, are, except in this 

 One regard of devitrification, not to be distinguished chemically or 

 microscopically from recent rhyolites, exhibiting fluidal or perlitic 

 or spherulitic structures. I have never been able to satisfy myself 

 as to the distinction, insisted on for some time, between propylites 

 and andesites, and I find that this is now repudiated by some of the 

 best American petrologists. In Hke manner there was no reason 

 for coining the barbarous term felsi-dolerite for the reception of some 

 of the lavas of the Lake-district. Chemically, the majority are typi- 

 cal andesites, a few are basalts rich in glass, very similar to those of 

 Tertiary age, and there are no other differences than such as are pro- 

 duced by lapse of time. In short, after a fairly exhaustive study of 

 *' felstones " and " trachytes " I may say that I am unable to recognize 

 any distinctions between the more ancient and the more modern, be- 

 sides those due to subsequent change, and that it is no more possible 

 to connect these with any single epoch in geology than grey hair in 

 the human subject with any one year of life. 



The strongest arguments in favour of the division have been 

 derived from the " mica-traps," the nepheline and the leucite rocks, 

 * Geol. Mag, Dee. 2, vol. x. 



