60 



tion of the cliff soon changes, however, and the dike then enters the hill 

 behind, and is lost. The sandstone having been completely worn away 

 from the eastern or seaward face of the dike by the action of the sea, 

 when it covered the present terrace, a very large surface of the side of 

 the dike is laid bare, and thus the structure is exhibited in the most 

 satisfactory manner, as shown in the annexed cut representing the 

 dike as seen on the east side. 



These dikes, as already remarked, are prismatic across, not vertically 

 as in the overlying trap. The cause has been alluded to in a former 

 article (Article 31); the imprisoned caloric of the submarine lava- 

 stream passed off perpendicularly to the cooling surfaces, and hence 

 the divisional planes are also perpendicular to these surfaces, or the 

 dike is prismatic across. In this case the prisms are mostly penta- 

 gons and hexagons. The dike is composed of greenstone, and is about 

 fifteen feet wide. 



THE CUMBKAYS. 



41. These islands are composed of old red sandstone similar to 

 that of the mainland adjoining, broken through and overlaid by 

 various traps, chiefly greenstone. The cliffs at the lighthouse on 

 the lesser Cumbray are composed of sheets of prismatic greenstone 

 rising above one another in successive terraces, having a dip N.E. or 

 contrary to that of the sandstone and trap of Garroch head, and 

 hence suggesting the existence here of a great fault in the strata, 

 giving origin to this main entrance into the proper estuary of the 

 Clyde. The chief interest of the islands is in their whin dikes. These 

 alter the strata remarkably: the sandstone has been fused by them, 

 and reconsolidated into a substance closely resembling a dark quartz 

 rock ; simple induration is induced at a greater distance from the dike. 

 Many of the dikes also stand out boldly from the adjoining sand- 

 stone, which has been worn away on either side as in the case men- 

 tioned in the last article, the amount of wearing in the ancient 

 tideway, compared with that in the present, affording a rude measure 

 of the time during which the sea remained at the higher level. 

 Attention was first called to these remarkable dikes by Mr. Smith of 

 Jordanhill. The best example is seen a little to the east of the 

 entrance of Millport harbour, where a large dike rising from the sea 

 level like a huge wall runs far up along the hillside, raised as if by 

 art above the surface of the fields. On the shore of the mainland 

 opposite, another dike, having the same direction and apparently a 

 prolongation of this, stands out in the same manner from the surface 

 of the sandstone. The similarity of the two masses of rock and their 



