352 OTHER CLYDE ISLANDS. 



in great successive sheets, these trap rocks give a terraced 

 and ridgy structure to the island; and, rising south-west, 

 present steep, abrupt cliffs towards the sea, while they 

 decline in succession in a N.E. direction, or contrary to the 

 inclination of the rocks of the Garrochhead. This is no 

 doubt due to the original inclination of the surface over 

 which the submarine lava streams were spread out; but that 

 inclination is most probably due to a great fault in the sand- 

 stone, on which the sea, working more successfully along a 

 fracture, opened this main entrance into the proper estuary 

 of the Clyde. 



There is a very interesting variety among the trap rocks 

 here, and they often approach the form of perfect columns. 

 They differ from the Garrochhead traps; and are much more 

 closely related in mineral structure to those of the mainland. 



Mr. Smith of Jordanhill has remarked, that in exact 

 correspondence with the greater hardness of the rock, the 

 terrace in front of the old sea-cliff is narrower than in Great 

 Cumbrae; and he finds glacial striae marked upon the surface 

 of the terrace parallel to the cliff, and extending, still unworn, 

 beneath the present level of the sea an observation of the 

 utmost importance. See our remai'ks in regard to the 

 terraces at lorsa water-foot (Newer Pliocene GeoL, p. 144; 

 and Jour. GeoL Soc., 1862). 



The small keep, on a peninsula on the eastern shore, is one 

 of a range of watch-towers erected along the Clyde shores 

 during the wars of the Edwards. 



PLADDA. 



\ 



153. This islet, we have seen already, is attached to Arran 

 by a whin dike, over most parts of which there is broken 

 water at low tide. The island consists almost wholly of a 

 dark-coloured trap rock, the sandstone foundation appearing 

 only on the north-eastern shore. 



