186 



IGNEOUS CAUSES. 



riods, 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- 

 rials 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 grea^. 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 WHICH FORM ROCKS (CONTINUED). 



Igneous Causes. 



Volcanic Action. — Definition. — Leibnitz's Theory. — Charle* 

 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.— 

 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- 



