314 Geological Society: — 



sites of those vents. In some cases the mineral characters of the 

 strata have been greatly altered, while their fossils have been occa- 

 sionally wholly obliterated by the action of these same igneous forces 

 during Tertiary times. 



In every case the survival to the present day of the patches of 

 Secondary rocks can be shown to be due to a combination of most 

 remarkable accidents ; and a study of the distribution of the frag- 

 ments shows that the formations to which they belong originally 

 covered an area having a length ot 120 miles from ~N. to S., and a 

 breadth of 50 miles from E. to W. But it is impossible to doubt 

 the former continuity of these secondary deposits of the Hebrides 

 with those of Sutherland to the north-east, with those of Antrim to 

 the south, and with those of England to the south-east. From the 

 present positions of the isolated fragments of the Mesozoic rocks, 

 and after a careful study of the causes to which they have owed 

 their escape from total removal by denudation, the author con- 

 cludes that the greater portion of the British islands must have 

 once been covered with thousands of feet of secondary deposits. 

 Hence it appears that an enormous amount of denudation has gone 

 on in the Highlands during Tertiary times, and that the present 

 features of the area must have been, speaking geologically, of com- 

 paratively recent production — most of them, indeed, appearing to be 

 referable to the Pliocene Epoch. 



The alternation of estuarine with marine conditions, which had, 

 on a former occasion, been proved to constitute so marked a feature 

 in the Jurassic deposits of the Eastern Highlands is now shown to 

 be almost equally striking in the Western area ; and it is moreover 

 pointed out that the same evidence of the proximity of an old 

 shore-line is exhibited by the series of Cretaceous strata in the West. 



The succession and relations to one another of the series of 

 deposits, now described as occurring in the Western Highlands, is 

 given in the following Table : — 



Miocene Volcanic and Intervolcanic Rocks. 



Unconformity. 



i ( i- 



1 1 2. 

 "8 | 3. 



Estuarine clays and sands with coal 



Maj 



. thicknesses, 

 feet. 

 20 + 



White Chalk with flints (Zone of Belemnitella mucro- 

 nata) 



10 + 

 100 



Estuarine Sandstones with coal 





6 l, 4. 



Upper Creensand beds 





60 









Unconformity. 





f 5 



Oxford clay 





? 



o 



6. 



7. 



8. 



9. 

 10. 

 11 



Great Estuarine Series 





1000 

 400 









100 



£h 



Middle Lias 





500 



t-s 







400 





Infralias • 





200 



12. 







1000+ 









Unconformity? 



Carboniferous strata (Coal-measures). 



Unconformity. 



Old Gneiss Series and Torridon Sandstones. 



