J. W. JUDD ON THE SECONDARY ROCKS OF SCOTLAND. 675 



markable patches of Secondary strata, on the singular isolation of 

 which I have already spoken ; and the diagram section, PI. XXXI. 

 fig. 2, will sufficiently serve to illustrate the peculiar manner in 

 which the patches of Secondary strata are exhibited in these re- 

 markable outliers. 



Around the smaller volcanos of Rum and Ardnamurchan tho 

 destruction of the basaltic plateaux has been carried still further, 

 and only a few fragments in the island of Rum, with those of 

 Eigg and Muck and that in Ardnamurchan, remain to tell of their 

 former vast extent and thickness. In the cases of the island of Eigg 

 and of the peninsula of Ardnamurchan it is clear that this work of 

 destruction by denudation would have been carried much further but 

 for the fact of the protection to the Miocene basalts afforded by the 

 solid masses of newer lavas erupted in Pliocene times — those, namely, 

 which form the lofty ridge of the Scur in the former case and the 

 mountain-pass of Beinn Shiant in the latter. 



Now the volcanic action which took place during the Tertiary 

 epoch has operated in three different ways in effecting the preser- 

 vation of relics of the Secondary strata in the Western Highlands. 



Eirst, by burying these strata under deposits of lava, the succes- 

 sive streams of which were in some cases piled upon one another to 

 the depth of 2000 feet. By this means the destructive effects of 

 denuding forces on the soft Mesozoic deposits has been arrested 

 during the enormous periods required for the removal of these vast 

 masses of lava. 



Secondly, by entangling portions of the Secondary strata between 

 sheets of igneous matter, intruded in a fluid condition into their midst, 

 usually in planes coinciding approximately with those of the bedding 

 of the sedimentary masses. As these intrusive sheets usually consist 

 of rocks of highly crystalline character and intense hardness, they 

 frequently display most remarkable powers of resistance to the 

 denuding forces; and the patches of Secondary rock enclosed be- 

 tween them — which have often themselves been rendered less suscep- 

 tible to disintegrating action through contact metamorphism — are 

 preserved with them. 



Thirdly, by enclosing fragments of the Secondary rocks after they 

 were blown from eruptive vents in the piles of agglomerate and tuff 

 constituting parts of the ancient Tertiary volcanic cones. 



The frequency with which the traces of Secondary strata are 

 found appearing from underneath the lava-cliffs of the great basaltic 

 plateaux of the Hebrides is very remarkable. As in many of these 

 instances, however, the beds in question are almost wholly concealed 

 by taluses, while in others they are exposed to view only at dead low- 

 water, and in some cases they must be examined during favourable 

 concurrences of spring-tides with certain winds, their study is 

 attended with great difficulties ; but it is almost certain that, were 

 we able to reach the base of the series of lavas at other points, we 

 should find many more exposures of the Secondary strata. In short, 

 it is clear that those great movements of subsidence which, as we 

 have seen, have operated so powerfully in the Western Istes, have 



