EVOLUTION OF IGNEOUS ROCKS. 201 



illusion vanishes, as Yon Cotta clearly saw ; for instead of finding 

 other zones, or magmas of the earth's substance tapped by ancient out- 

 bursts of the secondary and primary periods, we still meet with rhy- 

 olites, basalts, trachytes, and andesites ; but in an order of succession 

 in time which bears no relation to their occurrence in Richthofen's 

 tertiary system. We therefore are compelled to abandon the hypo- 

 thesis that volcanic rocks are derived from the original products of an 

 igneous fusion of the earth ; and no law has ever been suggested 

 which would on this hypothesis, show that the order of their succes- 

 sive appearance in the Primary period could be harmonized with their 

 order in the Tertiary period. It is an attempt not to disentangle and 

 explain the problem, but to escape from it by assuming results sup- 

 posed to follow from original igneous fusion of the earth. 



Chemical Hypothesis of Davy and Daubeny. Another view 

 which cannot be altogether ignored is the chemical theory, originated 

 by Sir Humphrey Davy, modified and supported by Daubeny, and 

 combated by Bischoff. This conception, also starting from an original 

 igneous fusion, rested on the hypothetical idea, that the interior of 

 the earth consisted of the elements in an unoxidised condition ; and 

 as water and the atmosphere obtained access to these substances from 

 the earth's surface above, so chemical changes were assumed to be 

 developed, which gave rise to volcanic energy. We need not now 

 delay to examine this view in detail. The potency of chemical action 

 to produce important changes in rock materials, and possibly in the 

 relative level of land, may be regarded as established ; but when \ve 

 remember the demonstrated continuity of volcanic outbursts with 

 plutonic phenomena, we seek in vain for any evidence which would 

 allow us to believe that the internal heat of the earth, crystallisation 

 of igneous rocks, and eruption of lavas, are exclusively or even prima- 

 rily chemical processes. 



Hypothesis of Igneous Evolution. Finally, there is the hypo- 

 thesis which regards all igneous rocks as metamorphosed from ancient 

 sedimentary strata, in the manner indicated in a previous chapter. 

 This may be termed the hypothesis of igneous evolution, which sup- 

 plements the hypothesis of organic evolution. For since the most 

 ancient fossiliferous rocks contain highly-organised groups of animals, 

 and even genera which still survive, we are compelled to believe, if 

 the hypothesis of organic evolution is true, that strata immensely 

 more ancient than those preserved must have existed, and represented 

 long eras of past time, during which the earlier steps of organic evo- 

 lution took place. And since no such rocks have been found, a belief 

 in organic evolution enforces the conviction that such strata were 

 metamorphosed, worn into sediments once more, and metamorphosed 

 again, with many repetitions of the process, effacing from the earth 

 all traces of its ancient life in times antecedent to the Primary era. 

 The evidence on which this conclusion rests is essentially palseonto- 

 logical; but it will be sufficient to restrict ourselves now to the 

 observed facts of igneous outbursts, and see how far the origin of 

 igneous rocks can be explained on this hypothesis. 



