1883-1884.] 267 



denudation of the Scottish Highlands during very recent geo- 

 logical periods, demonstrated by the occurrence of numerous 

 post-Miocene faults, having downthrows of various amount, up 

 to nearly 2000 feet, the effect of which upon the surface has 

 been entirely masked by post-Miocene denudation."* Even all 

 this it is obvious may be insignificant to the denudation effected 

 over tracts which are now sea, or to that which has acted over 

 land surfaces which no longer present any shreds of the basalts 

 except the dykes through which they welled up. Is it incon- 

 sistent, I would ask, in face of such stupendous results, to stretch 

 back the time during which the causes may have operated to 

 the remotest possible limits ? 



We thus have next to inquire what these utmost possible 

 limits may be, and how far back in geological time we may 

 place the first convulsive throes of eruptions of such appaUing 

 magnitude. 



The basalts in Ireland rest upon the upper chalk, and are, 

 therefore, not so old as this part of the Cretaceous series. They 

 must, indeed, be considerably younger, for on the north the 

 surface of the chalk on which they rest is bared and denuded, 

 and probably sub-marine, while to the south and east the thick 

 capping of loam and flints and the piped surface that intervene, 

 denote a long-continued terrestrial exposure prior to the arrival 

 of the lava. In Scotland, moreover, according to Judd, the 

 upper chalk, or zone of BeJemnitella miicronata^ is covered by 

 intervening estuarine and coal-bearing strata, which, though but 

 20 feet thick, suffice to negative the idea that the basalts were 

 in any way contemporaneous with any part of the true Cre- 

 taceous series of Britain ; though it is quite possible that 

 volcanic activity may have been developing towards its close. 



Between the newest Chalk of England, however, and the 

 oldest Eocene, there is an enormous interval. The magnitude 

 of the break has led to the Chalk being classed as a Secondary, 

 and the Eocene as a Tertiary rock. In the interval, formations 

 were accumulating in America and New Zealand, of immense 



* Judd, ^uart. Journ. Geol. Soc, vol. xxxiv., p. 699. 



