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CHAPTER XIV. 

 THE NATURE AND ORIGIN OF IGNEOUS ROCKS. 



WHEN we contemplate the products of igneous energy, as seen in 

 lavas, dykes, and the materials of mountain masses, we are impressed 

 by their diversity. From Vesuvius alone Von Buch distinguished 

 eighteen distinct principal kinds of lava, besides many varieties ; and 

 the number of minerals found on this volcano forms a considerable 

 fraction of those of the whole world. It has always been difficult to 

 account for the variety of igneous rocks ; and hence many hypotheses, 

 from time to time, have been suggested in elucidation of observed 

 facts. There are three principal views, all supported by a large 

 amount of evidence, which are more or less worthy of attention, since 

 they have been adopted by able observers. 



Hypothesis that the Earth's Crust is formed of Layers of 

 Different Igneous Rocks. First, there is the hypothesis that all 

 igneous rocks are derived from the more or less liquid interior of the 

 earth. In its crude form, as stated by the earlier writers, this view 

 had but little to recommend it, beyond its obvious simplicity. But 

 when the observed facts of basaltic lavas being in many cases poured 

 out after the trachytic lavas, as determined by Bunsen and earlier 

 writers, became generalised into the hypothesis of the existence beneath 

 the surface of two shell-like layers of rock material, the doctrine ac- 

 quired importance ; for the outer layer was assumed to be formed of 

 highly silicated or acidic rocks, of lower specific gravity, and more 

 perfectly oxidised, than the deeper-seated basic layer, which was 

 inferred to have been extruded after the acidic layer had been poured 

 out and cooled. 



Subsequently, when Von Richthofen divided volcanic rocks into 

 five groups, it became necessary to admit the idea of five of these 

 magmas, forming successive shells or onion coat-like layers of the 

 original fluid earth, which had been successively erupted. And when 

 we bear in mind that this classification of lavas into propylite, ande- 

 site, trachyte, rhyolite, and basalt, represents the order in which they 

 are superimposed upon each other in Hungary, Transylvania, North 

 Germany, and the great volcanic region of North America an order 

 which experienced geologists have found never to vary there is an 

 a priori case in favour of thus accounting for the diversity of igneous 

 rocks. But we need to bear in mind that this sequence, even if it 

 should hold good for larger areas than those in which it has been 

 established, is, at best, an order which characterises certain eruptions of 

 the tertiary period ; and the moment we go further back in time, the 



