670 J. W. JTJDD ON THE SECONDAKY EOCKS OF SCOTLAND. 



downthrows of various amount, up to nearly 2000 feet, the effect of 

 which upon the surface has been entirely masked by post-Miocene 

 denudation. 



He who wishes to obtain some faint idea of thevastness of the periods 

 of time represented by our great geological systems, cannot do better 

 than go to the Western Highlands. After taking careful note how 

 little has been accomplished by meteoric agencies since the Glacial 

 epoch in obliterating its characteristic markings, let him study the 

 enormous changes which have clearly resulted from the action of the 

 same forces operating during and subsequently to the Pliocene period. 



That during all the geological periods, from the Carboniferous to 

 the Cretaceous inclusive, a very large part of the Highland districts 

 was submerged, and formed areas of deposition, I think it is impos- 

 sible to doubt ; but that some portions of that Highland region did, 

 during those long periods, exist more or less continuously as islands, 

 we shall see in the sequel that there are good grounds for believing. 

 Yet so uniformly similar is the succession of life-forms during the 

 Mesozoic deposits, as exposed in Central Germany, in Northern 

 France, in England, Scotland, and Ireland respectively, that it seems 

 to me impossible to doubt that the Jurassic and Cretaceous deposits 

 of all those areas were accumulated in the same sea — one in which 

 the diffusion of the forms of life was not impeded by the existence 

 of any great continuous barrier of land. 



So far as I have been able to ascertain, there is only one exception 

 to the statement that has been made that the Secondary strata of the 

 Western Highlands owe their preservation to the agency of the 

 Tertiary volcanic outbursts. This single exception occurs in the 

 case of the Poikilitic strata of Gruinard Bay — where the soft unmeta- 

 morphosed strata in question have escaped total removal by denu- 

 dation, in consequence of the action of the same causes as have, on 

 the eastern coast of Scotland, effected the preservation of the Secon- 

 dary strata. The beds of Gruinard Bay constitute the most northern 

 exposure of Secondary rocks on the west coast of Scotland ; they 

 occur at a considerable distance from any of the great Tertiary vol- 

 canic centres ; they are not traversed by the dykes, nor do they 

 exhibit any evidence of having been ever covered by the lava-currents 

 of that period. The beds in question, indeed, appear to owe their 

 escape from destruction by denuding forces solely to the fact that 

 they have been let down amidst the hard metamorphic rocks of the 

 district by a great trough-fault, as shown in the following section 

 (fig. 1), which I have constructed from data obtained by Dr. 

 Taylor Smith and myself, and laid down on the excellent Admiralty 

 chart of that part of the coast. 



In all other cases of the preservation of patches of Secondary 

 strata on the west coast of Scotland, the presence of Tertiary volcanic 

 masses above or between them has evidently played a very important 

 part in bringing about the result. In not a few instances, however, 

 great movements and dislocations of the strata themselves have 

 occurred, either before, during, or subsequently to those volcanic 

 outbursts ; and these ; it is clear, have contributed, sometimes in a 



