254 RASA Y. GEOLOGY. 



lowest shale and limestone of Rasay. It must be a 

 subject for future inquiry whether that limestone is 

 analogous to the mountain limestone of English geologists, 

 or whether it be the lowest of the lias series. But 

 there can be no doubt that the upper beds of this 

 series in Rasay, the organic bodies of which have been 

 recently described, belong to that formation ; although 

 it rarely happens that the beds of lias are reduced to 

 such a degree of tenuity. 



The sandstone which follows possesses a very marked 

 character, and is known in other places to succeed or 

 accompany these latter substances, exhibiting also the same 

 gradations of composition which here take place at the 

 poirt of transition between those two sets of beds. Were 

 any other evide'nce of this succession required, it would be 

 found in Sky ; on the north-eastern shore of which island 

 the whole series of the lias will hereafter be distinctly 

 pointed out; and in such a position as to leave little 

 doubt of its being a part of the formation of Rasay, sepa- 

 rated only by the intervening sea. To that description I 

 shall refer the reader for such further remarks as may be 

 required, and proceed to consider the porphyry and trap, 

 as the uppermost rocks occurring in this island. 



As the sandstone is found occupying the eastern side 

 of the southern division of Rasay, so the porphyry is 

 limited to the western, or to that part immediately opposed 

 to Sky, where corresponding rocks are found in a similar 

 situation occupying a considerable extent. It is equally 

 remarkable that this porphyry is no where found among 

 the gneiss, ceasing indeed before the red sandstone appears. 



The scattered and irregular position of this rock renders 

 it impossible to define its boundaries, either by words 

 or on the map. I have however distinguished it by a 

 line, which may be taken as an average of its extent; 

 marking those places where the most remarkable junctions 

 with the secondary strata take place, as well as the size 

 of the drawing permitted. 



