107 



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 recti- 

 linear 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 surface, 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 being now completely 

 arrested in such situations. But dikes placed under circumstances 

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

 prismatic structure is the same (Arts. 31, 40), the chemical compo- 

 sition 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. 

 57), " on the decay of trap boulders, I found that certain varieties 

 of that rock are rapidly changed by the action of water ; lime and 

 magnesia being dissolved out, the iron converted into a peroxide, and 

 a crust formed on their surface, which is brittle and easily abraded." 

 Mechanical and chemical differences have thus both to be considered, 

 as well as the relative powers of resistance of the dike and the con- 

 taining rock. 



A remarkable group of dikes occurs under the east end of the high 

 cliffs, near the point where the shore bends southwards. One of 

 these is the broadest dike of greenstone on this coast. Its general 

 breadth is twenty-five feet, but it widens at one place to forty feet. 

 The sandstone is rendered very hard and quartzose to the distance 

 of several feet. The range is 47 W. of N., and the inclination east 

 at a small angle. A deep fissure marks the course of the dike up 

 the front of the cliff. This dike is noticed by Play fair, in his Illus- 

 trations of the Huttonian Theory ( Works, vol. i., Art. 266), as produc- 

 ing a marked change on the sandstone, and as indicating the relative 

 durability of the two rocks. A little east of this dike is another, nine 

 to twelve feet wide, inclined to the west, and ranging 18 E. of N. It 

 offers no remarkable appearances ; but the next dike east of it, though 

 but seven feet wide, alters the sandstone more than any other of the 



