J. W. JTTDD ON THE SECONDARY ROCKS OF SCOTLAND. 683 



Neocomian age were likewise deposited in the same area, and at the 

 same time to remember the evidence which there is of such enormous 

 denudation having swept the district again and again during successive 

 geological epochs ■; let him, too, reflect on the fact that beds deposited 

 during the period in question would have the worst possible chance of 

 surviving to the present day, in consequence of that wide-spread up- 

 heaval and erosion which preceded the Upper Cretaceous epoch ; and 

 keeping all these considerations before his mind, he will certainly 

 hesitate before accepting the negative evidence in the case as final, 

 and concluding from the absence now of any discoverable traces of 

 the Upper Oolitic and Neocomian formations that deposition was not 

 going on in any portion of this area during the periods in question. 



On the enormous amount of denudation which must have taken 

 place during the latter half of the Tertiary period, I have already 

 dwelt in the earlier portion of this paper. In a former communica- 

 tion I showed how vast were the results effected in the removal of 

 great thicknesses of rock during the earlier portions of the Tertiary 

 period, before and during the long succession of eruptions from the 

 great Hebridean volcanos ; and now I must touch briefly on the 

 great destruction of stratified masses which must have occurred in 

 the period preceding the formation of the Cenomanian strata. The 

 great unconformity between the Upper Cretaceous and all the older 

 deposits on which it is found resting is a very striking feature both 

 in the south of England, the Yorkshire area, and the north-east of 

 Ireland ; more than this, it is, as Professor Suess of Vienna has so 

 well pointed out*, equally well-marked in the Alps and almost every- 

 where within the European area. But this remarkable uncon- 

 formity and overlap is nowhere better exhibited than among the 

 exposures of the Secondary strata* in the Western Highlands and 

 Hebrides, where the Scottish Upper Greensand is seen overlapping 

 all the older formations and resting sometimes on the Jurassic, at 

 others on the Poikilitic, and others, again, on the Older PalEeozoic 

 gneissic rocks. That this unconformity and overlap has been 

 brought about by grand and widespread subterranean movements — 

 elevation, denudation, and subsidence succeeding one another during 

 long periods — no geologist can for a moment doubt. This being the 

 case, we are certainly, as I have already hinted, prepared for, and 

 the less surprised at, the absence of those members of the Mesozoic 

 series that would be deposited immediately before the incidence of 

 this epoch of change, and would consequently be the first to suffer 

 during its revolutions. And in a district where only minute patches 

 of older deposits have escaped destruction in consequence of a con- 

 currence of accidents (of such remarkable character in themselves 

 and occurring in such singular combinations), it would be idle for 

 the geological interpreter to express surprise, however keen may be 

 the regret he feels when he finds certain chapters of the record alto- 

 gether missing. 



* See his * Entstehung der Alpen.' Vienna, 1876. 



