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



hope of ever being able to collect from them (such is the imperfect 

 maimer of their exposure) the interesting remains of plant- and 

 animal-life which would be so invaluable to him in enabling him to 

 trace the ancestry of some of the Tertiary and recent types of life. 

 The discussion of the question concerning the probable existence of 

 land during the Cretaceous and later periods in the area now occu- 

 pied by the Atlantic, and the more general problem of the possible 

 permanence of the position of the great continents during vast 

 periods of geological time, we reserve for a future occasion. 



It is impossible to avoid noticing, in passing, the singular fact 

 that in the north-western part of our archipelago both the Lower- 

 Silurian and Upper-Cretaceous strata lose their ordinary characters, 

 and show curious points of similarity to those displayed by the beds 

 of equivalent age in the North- American continent. 



The suggestion made some time ago, that the present Atlantic bed 

 was in some sense " continuous with" the ancient Chalk strata of Eng- 

 land, has been so^thoroughly disposed of on palaeontological and other 

 grounds already, that it is perhaps scarcely necessary even to allude 

 to it here. But if any trace of vitality still lingers in such a theory — 

 and it is the nature of error to produce perennial crops— it is surely 

 only necessary to point to the interesting fact, that in tracing the 

 Chalk strata towards the Atlantic area they are found to assume 

 estuarine characters and to afford the most unmistakable evidence 

 of the close proximity of land in that direction, in order to set the 

 question permanently at rest. 



The conviction, however, that makes itself most strongly felt from 

 a consideration of the details and arguments of the present part of this 

 memoir, is that of the extreme danger to the geological inquirer of 

 giving any important weight in his reasoning to negative evidence. 

 The conclusions of the present paper, which will be strengthened in 

 various ways by those which I hope to discuss on a future occasion, lend 

 the strongest grounds of support to the view that over the greatest 

 part of the British Islands, and far beyond that area, strata of various 

 ages, from the Carboniferous to the Chalk inclusive, were once very 

 widely spread, and have been removed by denudation during and 

 since Mesozoic times. As a corollary from this proposition, which 

 I believe that it will be possible to establish by the most unassail- 

 able arguments, we can scarcely help inferring that the reasonings so 

 frequently indulged in concerning the former limits of areas of 

 deposition in the past are of a very unsafe character, depending as 

 they do in so many instances on such purely negative evidence as 

 the absence of strata of a particular age from certain districts. We 

 are too apt, perhaps, to forget how enormous has been the work of 

 destruction of older strata in past times by denudation ; and inter- 

 esting as the new and isolated deposits of Carboniferous, Poikilitic, 

 and Cretaceous strata (which I have now described for the first time 

 as occurring in the Highlands of Scotland) are in themselves, yet 

 their value and importance to the student of geology will be greatly 

 enhanced if they be allowed to serve — as I believe they ought to do 

 — as monuments of the enormous destruction effected by denuding 



