ALLUVIAL AND DILUVIAL DEPOSITES. 129 



strata, that had never been indurated by drainage, 

 and the violent action of water upon them when 

 they were suddenly broken and forced upward, but 

 were still beneath the surface of the ocean. 



One of the largest mineral dikes known to exist is 

 the Cleveland Basalt Dike in England. This has 

 been traced 50 or 60 miles in length, and averages 

 about ten yards in width. There is also another 

 extensive dike near Durham, which throws down 

 the strata on one side of it 160 yards. It is worthy 

 of notice, that as dikes are generally impervious to 

 water, they obstruct its passage along the porous 

 strata, and occasion it to rise : hence it frequently 

 happens that numerous springs make their appear- 

 ance along the course of a dike, by which it may 

 be detected when there is no other indication of it 

 visible on the surface. Beds of coal are often 

 (Jrained by cutting through a dike, which prevents 

 the escape of the water. 



CHAPTER XII. 



ALLUVIA.L AND DILUVIAL DEPOSITES. 



Alluvial Deposites. — Bowlders. — Diluvial. — Mr. Lyell's Theory. 

 —Post Tertiary. — Modern Deposites : Terrestrial, Lacustrine, 

 Fluviatile. 



Alluvial beds* consist of the sand and soil 

 brought down by rivers, and deposited in their beds, 

 or scattered upon their banks, or carried into the sea 

 or into lakes, forming deltas at the mouths of riv- 



lava. Rents or fissures of some miles in length have been open- 

 ed on the sides of ^tna. 



* The term is derived from " alluvio" an inundation ; or from 

 ** aUuo" to wash. 



L 



