70 GEOLOGY OF ARRAN. 



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 containing rock. 



A remarkable group of dikes occurs under the east end of 

 the high cliffs, near the point where the shore bends south- 

 wards. 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 Playfair, in his 

 Illustrations of the Huttonian Theory (Works, vol. i. Art. 

 266), as producing 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 N. 18 E. 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 

 whole series. This is probably owing to the nature of the 

 rock, which, being a highly crystalline greenstone, must have 

 passed slowly from a state of fusion. The stratification of 

 the sandstone is obliterated through a space of seven or eight 

 feet, and this rock assumes the structure of a claystone. The 

 case is strongly in favour of the view often advanced, that 

 the Arran claystones are merely metamorphic sandstones. 

 Intersecting this dike is another, ten or twelve feet wide, 

 ranging N. 37 W., dipping E.N.E., and consisting of com- 

 pact fine-grained greenstone. It is sunk below the level of 

 the sandstone ; and, on this worn, depressed surface, there 

 rests a boulder of coarse-grained granite, estimated at about 

 thirty tons weight. Now, no force of surging waves here, 



