EXCURSION II. 69 



around are worn away, being softer or more ironshot. The 

 rock has thus a honeycomb structure, like what one often 

 sees in the worn corallines of the mountain limestone. Nume- 

 rous bowl-shaped cavities also occur, due, probably, to the 

 removal of imbedded quartz balls, or the grinding action of 

 these, by the movements of the waves, when once loosened 

 from their bed in the sandstone. 



Some general remarks have been already made (Art. 17) 

 on the dikes of this coast, and we shall now only notice the 

 individual cases of most interest. The whole number is 

 about sixty, and the direction generally between north-west 

 and north-east, a few running nearly east and west. Most of 

 them alter the strata of sandstone more or less. The great 

 majority are depressed below the level of the sandstone, 

 owing to the more rapid disintegration depending on 

 their structure, as already often noticed. They are of all 

 widths, from eight inches to forty feet, and the sides 

 are generally parallel, and the course rectilinear or 

 slightly undulating. Most of them traverse quite across 

 the rocky platform, and are continued into the cliffs, up 

 whose front they are seen to range, either level with the sur- 

 face, forming deep gashes, or projecting like walls. These 

 cliffs are the old sea margin, and are hollowed into caves along 

 their bases, and otherwise sea-worn to a considerable height. 

 The gashes in the cliffs were doubtless formed when the sea 

 stood higher ; the process is now in great measure arrested in 

 such situations. But dikes placed under circumstances ex- 

 actly alike do not waste with the same rapidity. Though the 

 prismatic structure is the same, the chemical composition 

 varies, as does also the internal texture, while the adjoining 

 sandstone varies also in its capability to resist decay. When 

 the alteration produced 'by the dike is great, the sandstone 

 will resist disintegration ; if the contrary is the case, the 

 sandstone may wear rapidly, and the dike project. " From 

 some experiments made several years ago," says Mr James 

 Napier (paper quoted in Art. 17), "on the decay of trap 



