22 Rev. A. Sedgwkr and Mr. MuncmsoN on the Geological 



secondary formations which are found in so many parts of the Hebrides, and 

 the northern coasts of Scotland. 



We hope in a subsequent communication to prove, partly by tlie lielp of the 

 Arran section, that the conglomerates above mentioned are the equivalents 

 of the old red sandstone, — that they are in a few places, and without the inter- 

 vention of the carboniferous order, surmounted by masses which probably 

 represent a portion of the newer red sandstone and its conglomerates, — and 

 that they thus exhibit, in strict conformity to the order of succession esta- 

 blished among- the English strata, a natural introduction to these superior 

 deposits which had been previously identified with the lias, and a large por- 

 tion of the oolitic series*. 



General strncture of the Island Arran. 



Having thus briefly explained tlie objects we propose to ourselves in the 

 present communication, it may be proper to describe in a few words the ge- 

 neral structure of the Isle of Arran, with a view of exhibiting the true con- 

 nexion between the formations which will be described in detail, and the 

 inferior rocks of the neighbouring districts. 



If a line be drawn from the south side of Brodick bay to Machrie water, 

 it will divide the island into two nearly equal portions, which are alike distin- 

 guished by their external features and internal structure. The hills which 

 rise in the interior of the southern region have generally neither a great 

 elevation nor a striking form, and are almost exclusively composed of different 

 varieties of trap. Near the coast we meet with extensive deposits of red sand- 

 stone, all apparently of one formation, and generally in an horizontal position ; 

 but overlaid, interrupted, and traversed by innumerable masses of trap, ex- 

 hibiting- every imaginable variety of combination. 



In the northern region, on the contrary, all the central parts are occupied 

 by lofty serrated mountains of granite, on the flanks of which repose a 

 number of distinct formations, the subordinate beds of which are in some 

 places much broken and contorted, and generally in a highly inclined posi- 

 tion. All these several formations, from the granitic nucleus to the newest 

 stratified deposits, are occasionally traversed by trap dykes, which perfectly 

 resemble some of the innumerable dykes of the southern portion of the island. 



Primary/ Slate of Arran, ^c. 



The northern, the western, and the southern slopes of the central moun- 

 tains of granite are succeeded by chains of hills composed of different varieties 

 of primary slate. On a part of the eastern slope, between North-Sannox and 



* Geol. Trans. New Scries, Vol. II, Part II. pp. 293, &c.— Part III. pp. 353, &c. 



