186 
lONEOUS CAUSES. 
yiods, springs in various parts of the world were as 
commonly impregnated as now with bituminous mat- 
ter, carried down, probably, by rivers into lakes and 
seas. 
It will, indeed, be easy to show that a large por- 
tion of the finer particles and the more crystallized 
.substances found in sedimentary rocks of different 
ages, are composed of the same elements as are now 
held in solution by springs, while the coarser mate- 
j-ials bear an equally strong resemblance to the al- 
luvial matter in the beds of existing torrents and 
rivers. 
We have thus noticed the most important exam- 
ples of the deposition of rocks from materials held 
in solution by various waters ; but, when compared 
with the great geological formations which exist on 
the globe, they are relatively unimportant ; though, 
as De la Beche observes, they teach us how such de- 
posites may, chemically, have formerly taken place. 
CHAPTER XVIII. 
AGENTS WmCH FORM ROCKS (cONTINUEd). 
Igneous Causes. 
Volcanic Action. — Definition. — Leibnitz's Theory. — Charles 
Darwin's Hypothesis.— Sir Humphrey Davy's Hypothesis. — 
Phenomena of Volcanic Eruptions.-— Quantity of Ejected Mat- 
ter. — Skaptar Jokul in Iceland. — Tomboro.— Submarine Vol- 
canoes. — Graham Island. — Etna. — Vesuvius. — Pompeii.— r 
Herculaneum. — Earthquakes. 
Having considered somewhat in detail those chan- 
ges which are wrought on the earth's surface by 
means of aqueous causes, we come now to contem- 
