PLADDA. GEOLOGY. 443 



PLADDA. 



PLADDA* is a very small and low island, more con- 

 spicuous for its light-house than for any thing interesting 

 in its geological features. It would scarcely have required 

 notice but for its connexion with Arran, and for the 

 blank which its absence would leave in the mineral 

 history of the islands of the Clyde. 



The stratified rocks, which must be considered as 

 forming the foundation of the whole mass, may be 

 seen for a small space towards the eastern extremity, 

 and they consist of a white sandstone accompanied by 

 limestone. There can be no hesitation in admitting 

 that these are continuations of the strata of Arran, and 

 it is therefore superfluous to describe them further. 



These strata are covered by a continuous bed of 

 trap, which forms by far the greater part of the island, 

 and reaches to the sea on almost all sides, so as to 

 exclude their prolongations from sight. The surface 

 of this bed is flat, and the edges present vertical and 

 somewhat prismatic fractures ; a feature common in Bute, 

 but rarely occurring in Arran. The nature of this rock 

 is in most places such, that, like many others described 

 in various parts of this work, it scarcely admits of 

 any very definite appellation, although a common sub- 

 stance, and found indifferently both in veins and over- 

 lying masses wherever it occurs. It cannot be considered 

 a greenstone, since it wants the requisite distinctness; 

 of parts ; although, like most of the trap rocks of this 

 character which are not decidedly basalt, it has frequently 

 been designated by that appellation ; a term rendered 

 vague only in consequence of its misapplication. The 

 rigid manner in which the term basalt has been limited, 



* Pladda, from Plade, a plate, Danish ; a very characteristic term, 

 See the Map of Arrau. 



