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108 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 8, 



practice ; the distinctions between the species of Cycladidce and 

 Unionidce respectively being of so delicate a character that the examina- 

 tion of numerous specimens of each species is necessary, combined with 

 a knowledge of the recent species of these excessively difficult tribes. 

 I cannot satisfy myself that any one of the Loch Staffin shells is iden- 

 tical with a Purbeck or Wealden species. This in the present state 

 of our knowledge was to be expected. 



A more curious result is that, after a close comparison of both Sir 

 Roderick's and my own specimens (now contained in the collection 

 of the Museum of Practical Geology), I cannot satisfy myself that, 

 with the exception of the Paludina conulus of Robertson, which is a 

 little Hydrobia identical with the unfigured Paludina mentioned in 

 Sir Roderick Murchison's Loch Staffin list, there is any one of the 

 Loch Staffin estuary shells identical with a Brora species. The little 

 Hydrobia above mentioned, however, appears to be undistinguishable. 



The succession of events indicated by the section I have described 

 is of no small interest, when considered in its bearing on the physical 

 geography of our area during the oolitic epochs. From the lias up 

 to the cornbrash, or beds probably equivalent to that stage in the 

 series of oolites, we have in the Hebrides, as was indicated by Mac- 

 culloch and proved by Murchison, a continuous sequence of marine 

 conditions, which, if I might venture to judge from the as yet im- 

 perfect evidence of the contained fossils, prevailed in a sea by no 

 means shallow. But at the termination of the deposition of the 

 middle oolitic strata, we have indications of most important changes, 

 and of the conversion of the bed of the Hebridean oolitic sea into an 

 estuarine and terrestrial area, which after a considerable lapse of time 

 became submerged under oceanic conditions and had a new series of 

 marine strata deposited upon it. 



If I read what I have seen aright, the plutonic phenomena which 

 accompanied these changes were not less interesting. The great and 

 thick sheet of imperfectly columnar basalt which has so wide an 

 extension in the island of Skye, and plays so important a part in the 

 formation of the magnificent scenery of its coasts, was the product of 

 a submarine eruption, which, if we regard this basalt as an overflow, 

 has its geological date marked to a nicety, having occurred at the 

 close of the middle and at the commencement of the upper oolitic 

 period. This vast cap of compact volcanic matter served to assist in 

 the consolidation of the muddy and sandy marine accumulations over 

 which it spread, and the Titanic throes of this region of eruptions 

 elevated the whole probably above the level of the ocean, and con- 

 verted a part at least of the sea-bed into dry land, the area of which 

 and of its fresh and brackish waters became again submerged, to be 

 again overwhelmed by the destructive outpourings of submarine vol- 

 canos ; their results we now see in the great and thick mass of 

 trap forming the line of hills constituting the chain of the Storr. 

 This trap has features distinct from those presented by the bed be- 

 tween the middle and upper oolites. It is in great part an amyg- 

 daloid, and its vesicular character may indicate the formation of it at 



