A]S^NIVEESARY ADDRESS OF THE PRESIDENT. 77 



all, it will be noted, rather rare and exceptional rocks. The first, it is 

 true, for long time appeared to be universally of Palaeozoic age, and in 

 England it is only lately that (in Devonshire) they have been found 

 to cut rocks so late as Carboniferous ; but !M. Barrels has described 

 kersantites*, which, in his opinion, are certainly Post-Cretaceous, and 

 most probably Post-Eocene. The majority of the nephcline rocks are 

 Post-Secondary. I am not aware that any nephcline-basalts have 

 as yet been identified prior to the Tertiary period, but although 

 many phonolites also belong to the latter, representatives of this 

 group of earlier date are by no means wanting. I will not press 

 the case of those remarkable masses of nepheline rocks which break 

 through Silurian limestones and so strangely interrupt the level plane 

 of the St. Lawrence Talley in the district near Montreal, although the 

 opinion of Canadian geologists is in favour of their antiquity, be- 

 cause I am not aware that there is any actual proof of their age ; 

 but I may remark that the nepheline-syenite, which one would 

 naturally, from its appearance, class with the Pre-Tertiary representa- 

 tives of this group, contains perfectly typical nepheline ; while some 

 of the phonolites closely resemble European phonolites of Tertiary 

 age. For a like reason I abstain from quoting the Wolf Rock of 

 Cornwall, and even the liebenerite-porphyries of the Fassa Thai, 

 although I think that the latter cannot well be later than some part 

 of the Mesozoic period. But in any case I am unable to recognize 

 more than a varietal difference in the so-called elaeolite-syenites, or 

 any real distinction between the nepheline which occurs in the 

 f oyaite of Portugal, considered by Dr. Sheibner to belong to the more 

 ancient eruptive series, and that in sundry rocks of Tertiary age. 



Little can be made of the restriction to Post-Secondary rocks of 

 such rare minerals as haiiyne and nosean, or of tridymite, which is 

 very possibly not an original constituent, and very easily overlooked. 

 It is of no avail to quote such minerals as tourmaline, topaz, beryl, 

 zoisite, andalusite, staurolite, cyanite, &c., as restricted only to 

 Pre-Tertiary rocks, because there is no evidence that any are proper to 

 igneous rocks, and most are distinctly minerals of metamorphic origin. 

 Muscovite also will not, T think, avail much, as its identity and 

 history are yet far from clear. The strongest point in favour of the 

 classification by geologic age can undoubtedly be made witJi the 

 leucite rocks, for there can be little doubt that all which have been 

 described are comparatively modern. Further I am not aware of 

 any good ground for suspecting that in any of the more ancient 

 rocks which have been microscojjically examined this mineral has 

 once been present, but has been replaced by pseudomorphs. Still 

 we must bear in mind that in this respect negative evidence is not 

 of great value ; for the mineral is an exceptional one, being peculiarly 

 rich in potash, and typical leucite rocks are very rare — so rare that for 

 some time no instance was known beyond the limit of Europe. 

 The experience also of Messrs. Fouque and Levy appears to me to be 

 significant. They melted together microcline and black mica, the 

 composition of the mixture being SiO^ = 40, Al,fi^ = 17, 'Fefi.^ = S, 

 * Eecherches sur les terrains anciens des Asturies, &c. p. 160. 



