678 J. W. JFDD ON THE SECONDARY ROCKS OE SCOTLAND. 



Equally certain is it that very considerable subterranean move- 

 ments and a vast amount of denuding action must have taken place 

 in the area in question during the Mesozoic period itself. This is 

 proved by the irregularities in thickness and the variations in the 

 development of the different members of the Secondary series, and 

 still more by the striking unconformities between certain of them. 

 Thus, as we shall point out more fully hereafter, the Upper Creta- 

 ceous strata overlap all the older rocks, being seen to rest on the 

 Middle Lias at Carsaig, the Lower Lias at Loch Aline, the Poiki- 

 litic at the Innimore of Ardtornish, and the Older Palaeozoic at 

 Gribun. 



Except in a very few instances, such as those of Strath, Applecross, 

 and Ardnamurchan, the Secondary rocks do not constitute any con- 

 spicuous areas capable of being represented upon geological maps ; in 

 the cases cited it is sufficiently manifest, too, that they are now only 

 exposed at the surface, in consequence of the removal of superincum- 

 bent lava-masses by denudation. The determination of the relations 

 of these isolated fragments of strata to one another by mapping their 

 lines of outcrop is therefore altogether out of the question in m©st 

 cases ; and in the few in which it is at all practicable the amount of 

 dislocation and igneous intrusion to which the strata have been sub- 

 jected renders the task one of much difficulty and, at best, very con- 

 siderable uncertainty. 



Continuous sections of these rocks are also rare, and where they do 

 exist, as on the east coast of Skye, are usually much interrupted, com- 

 plicated, and obscured by the intrusion of sheets and dykes of igneous 

 rocks among the strata. In the magnificent precipices of the east coast 

 of Raasay it is true that we have a section of over 1000 feet in depth 

 extending for a distance of several miles, one which is perhaps with- 

 out a parallel for its magnitude and variety in the British Islands ; 

 but even here there is, unfortunately, a very serious drawback to the 

 satisfaction felt by the geologist who studies this section ; for its very 

 magnitude militates against its value, since a very large portion of it 

 is utterly inaccessible, and those parts which can be reached are much 

 obscured by taluses. Even the smaller and more accessible sections 

 of the Secondary rocks exposed beneath the basaltic cliffs of the 

 Western Isles are also often, to a very great extent, covered and con- 

 cealed by fallen masses of the harder igneous masses which have 

 tumbled from above ; and in such cases the best exhibition of the 

 beds is often to be found in the reefs exposed along the shore at low 

 water. 



In addition to these hindrances to his study the geologist too fre- 

 quently finds that the patches of Secondary rocks, even when reached, 

 are capable of affording him very little information. Wherever, for 

 example, these masses of Secondary rocks happen to be in close proxi- 

 mity to one or other of the great volcanic centres, as in Ardnamurchan 

 and along the greater part of the south coast of Mull, the strata have 

 been metamorphosed almost beyond recognition ; they are in such 

 cases found to have assumed a more or less crystalline character, and 



