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 XVIIL 



AGENTS WHICH FORM ROCKS (cONTINUEd). 



Igneous Causes, ^ 



Volcanic Action. — Definition. — Leibnitz's Theory. — Charley 

 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- 



