169 



EXCURSION XIV. 



BY THE EASTERN SHORE TO DIPPEN AND LAG. 



77. WE enter now upon a region whose geological struc- 

 ture is more simple, and strata less disturbed. The rocks we 

 shall meet with are sandstone, felstone, and trap. The 

 sandstone is rarely conglomerate, and has a greater admix- 

 ture of argillaceous matter. Having been deposited as a fine 

 sediment in quiet waters, and afterwards consolidated by the 

 pressure of superincumbent ocean, aided by heat below, it 

 was affected by a general movement and extensively frac- 

 tured. Through the fissures thus formed streams of melted 

 matter were forced up from lakes of molten rock in " the 

 nether depths," and overflowed from hundreds of vents the 

 surface of the sandstone. Cooling under varying local condi- 

 tions, and having a slight local variation in its constituent 

 parts, this matter became felstone in one part and trap in 

 another, and differs from lava consolidated in the same way 

 simply in this, that it cooled, not in the atmosphere, as lava 

 does, but under a great pressure of incumbent ocean. Now, 

 it must be remembered that the granite and slates of the 

 north plunge beneath these sandstones, and must also have 

 been pierced by the streams of liquid rock. This escape of 

 molten trap, on a large scale, from beneath the granite, 

 relieved that rock and the whole southern district from an 

 enormous upward pressure; and hence the granite was not 

 raised in a solid form ; and hence, too, the contrasts between 

 the northern and southern divisions of the island. Liquid 

 matter poured out from beneath could not consolidate in the 

 form of steep ridge or beetling precipice; the disruptions 

 attendant on the elevation of the land, and the action of 

 currents, would impress upon the surface most of the 



