722 J. W. JTOD ON THE SECONDAKY ROCKS OF 8COTTx4If», 



The various sections in Skye and Raasay have been so fully de- 

 scribed, first by Sir Eoderick Murchison and afterwards by Dr» 

 Bryce and Prof. Tate, that it is not necessary to dwell further upon 

 the details exhibited by them. Locally they often exhibit a con- 

 siderable amount of metamorphism resulting from the action of the 

 numerous sheets and dykes of dolerite by which they are traversed. 

 The beds display in an eminent degree the character so frequently 

 exhibited by strata of littoral origin, and undergo great variations 

 in thickness and mineral character within very short distances. 



The greatly altered Inferior- Oolite strata of Strathaird are inca- 

 pable of throwing any new light on the general characters, order of 

 succession, or fossil contents of this formation. Their true position 

 in the geological series is, indeed, only rendered manifest by a study 

 of their relations to the Upper- and Middle-Lias strata. 



The Inferior-Oolite strata of Ardnamurchan are only one degree 

 less metamorphosed than those of Skye; but by carefully examining 

 some portions of the beds which have undergone less intense altera- 

 tion, we may, as already mentioned, detect here and there a fossil 

 enabling us to decide on the age of the beds. 



Southward from Ardnamurchan no traces of the Inferior- Oolite 

 strata have escaped being swept away by denudation till we reach 

 the well-known development of the formation in Yorkshire, where 

 they exhibit estuarine characters somewhat similar to those which 

 distinguish their Scottish equivalents. 



Similar estuarine conditions characterize the beds of the same age 

 in the place of their most northerly development in Britain, namely 

 Sutherland ; and in fact purely marine representatives of the forma- 

 tion are found only in the extreme southern portions of our island. 

 The formation is excessively variable in its thickness in different 

 areas, and over large portions of the British islands it is probable 

 that no beds of this age were deposited at all. But it is no less 

 clear that the few fragments left of its strata afford but very im- 

 perfect indications of the extent of country over which it was 

 originally deposited, extensive denudation both prior to the Creta- 

 ceous and subsequently to the Miocene having effected so much in 

 the destruction of the deposits of this period. 



f. The Great Estuarine Series. 



Beneath marine strata of Oxfordian age in the Eastern Highlands 

 there occur, as we have shown in the first part of this memoir, a 

 series of estuarine strata which contain the famous coal-seam of 

 Brora. In a somewhat analogous position in the Jurassic series of 

 the western coast similar estuarine beds also make their appearance. 



As we have already shown in the foregoing pages, the Lower 

 Oolites of the Hebrides include certain beds which exhibit evidence 

 of having been accumulated under littoral, occasionally passing into 

 freshwater conditions. But strata of far more pronounced estuarine 

 characters succeed the Lower Oolites of the Western Isles, and are 

 intercalated between them and the representative of the Oxford 



