ANNIVERSARY ADDRESS OF THE PRESIDENT. 7 5 



have never been able to find any solid ground, and will briefly indi- 

 cate the reasons why I venture to dissent from several eminent 

 authorities. At the outset, we naturally feel some surprise that the 

 commencement of the Tertiary period should coincide with so 

 marked an epoch of change in the history of petrology, so that the 

 igneous rocks, like the mammalia, should be en pleine evolution after 

 the close of the Secondary period. But we may further ask, Was 

 there any long pause, any universally definable limit, between the 

 two periods ? Did the curtain fall for an interval between two acts 

 of the drama of life played on the world's stage? Granted that 

 Tertiary can be sharply defined from Secondary in Britain, or even in 

 parts of Europe, can that line be drawn everywhere ? Palocontolo- 

 gists, geologists in general, will, I think, accord in returning us a 

 negative answer. Still, admitting the impossibility of adopting any 

 very hard and fast line, there is yet a possibility that a certain 

 " evolution " may exist among the inorganic products of the earth, 

 and that the older may be distinguishable from the newer rocks. 

 Let us then inquire how far this idea is in accordance with facts. 



The older rocks, of course, are more likely to have undergone 

 mineral changes during the vicissitudes of their longer history. The 

 less stable minerals will have disappeared, and their constituents will 

 be represented in more stable forms. Olivine will have been changed 

 into serpentine and iron peroxide : augite and diallage into some 

 form of hornblende, or all these will have been replaced by viridite 

 or chloritic minerals : felspars will have been replaced by zeolites or 

 other alteration-products ; their materials may have been employed in 

 the composition of tourmaline and epidote, and the like. If the rock 

 has had a glassy matrix, this may have been devitrified. In short, 

 an ancient rock, like a living creature, can hardly fail to exhibit 

 signs of old age. Thus we naturally expect to find such rocks as 

 serpentine and diabase among the older formations, and should 

 hardly expect that a Pre-Cambrian or an Ordovician lava would b« 

 absolutely identical with one emitted during the latest geological 

 epoch. Fiirther, as we hold that the more coarsely crystalline rocks, 

 especially when members of the more acid division, have solidified 

 beneath the pressure of superincumbent rock-masses, we should 

 expect such rocks as granite to be usually of ancient date, not be- 

 cause a modern granite may not exist underground, but because it has 

 not yet been exposed to view by denudation. It must, however, be 

 remembered that there seems no reason to doubt the Tertiary age of 

 some of the granite of the Inner Hebrides; certain Alpine granites also 

 seem to me to be most probably Post-Secondary ; at any rate I have 

 seen in the West-central Alps perfectly typical granite cutting Lower 

 Cretaceous strata, and I know of no indications of disturbances in 

 that region until the Tertiary period had begun. Tertiary granite 

 is also said to exist in the island of Jamaica. Some of the Carbo- 

 niferous basalts of Scotland are admitted to be undistinguishable 

 from those of Miocene age ; most of them only differ by reason of 

 subsequent mineral change. Eestore the rock (and that it can be 

 restored admits, I think, of no reasonable doubt) to its original con- 

 dition, and your diabase resumes its place in the ranks of the normal 



