728 J. W. JUDD ON THE SECONDARY ROCKS OF SCOTLAND. 



far as they can be now ascertained, I hope to discnss on a future 

 occasion. 



' D, The Cretaceous System. 



The Cretaceous strata of the Western Highlands, although of no 

 great thickness, are of surpassing interest. The fact of their pre- 

 sence in the area in question is one which, as we have already 

 pointed out, is full of significance to the geologist who seeks to 

 realize the vastness of the changes in physical geography of the 

 Highland districts which have taken place during successive geolo- 

 gical periods. But when the peculiar and anomalous character 

 presented by these beds in the Scottish area comes to be fully 

 appreciated, their important bearing on many great questions of 

 theoretical geology must be apparent to every one ; and the sugges- 

 tiveness of the facts is enhanced when we compare the Scottish 

 Cretaceous beds with their equivalents in other parts of Northern 

 Europe and America. 



As we have already pointed out, no vestiges of Neocomian strata 

 have as yet been found in any part of the Western Highlands ; but 

 we have sufficiently insisted on the danger of accepting the negative 

 evidence as being conclusive against such strata having ever been 

 deposited within the district. From a careful consideration of all 

 the facts of the case, I believe there are strong grounds for the pre- 

 sumption that very large areas in the Highlands were once covered 

 by strata of Upper-Oolite and Neocomian age, the former being cer- 

 tainly represented by deposits of great thickness and importance. 



The Upper Cretaceous strata, though represented at the present 

 day by such minute and inconspicuous patches, must once have had 

 a wide range, and in places probably attained a considerable thick- 

 ness. It is clear, from the abundance of chalk-flints in certain 

 Miocene inter-volcanic deposits, such as those of Ardtun and the 

 Innimore of Carsaig, that very extensive masses of Chalk must have 

 been destroyed by denudation in order to supply the quantities of 

 flints accumulated in those deposits. It is probable, indeed, that 

 very considerable masses of strata of Cretaceous age still exist buried 

 under the great lava plateaux of Mull and Morvern. 



The Upper Cretaceous strata of the Western Highlands rest 

 everywhere unconformably upon the older rocks. Thus at Carsaig 

 they are seen reposing directly upon the Middle Lias ; at Loch 

 Aline they overlap successively the Lower Lias, the Infralias, and 

 the Poikilitic ; while at Gribun they appear to lie in part on Poiki- 

 litic strata, and in part on the old gneissic rocks, which are presumed 

 to be of Lower Silurian age. 



The succession of deposits belonging to the Upper Cretaceous in 

 the district of the Western Highlands is very peculiar ; for we there 

 find the strata of this age assuming those littoral and estuarine con- 

 ditions which are so strikingly exhibited by the underlying Jurassic 

 beds. As might be expected under such circumstances, the Creta- 

 ceous beds exhibit very rapid and remarkable changes in thickness 



