ALLUVIAL AND DILUVIAL DEPOSITES. 129 
Strata, that had never been indurated by drainage, 
and the violent action of water upon them when 
they were suddenl}^ 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 
drained by cutting through a dike, which prevents 
the escape of the water. 
CHAPTER XIL 
ALLUVIAL AND DILUVL^lL 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 JEtna. 
* The term is derived from " aZZwwo," an inundation ; or from 
" aZ/wo," to wash. 
