ON THE FORMATION OF CRYSTALLINE ROCKS, 241 



Dufrenoy showed that these transformations were due to the in- 

 tercalation of granitic masses; that they existed throughout the en- 

 tire extent of the chain, and that they affected formations of different 

 ages, up to and comprising the chalk.* He afterwards showed that 

 accumulations of iron ore in the neighborhood of granite also took 

 place after the formation of the chalk and in consequence of the up- 

 heaval of that chain, t Thus the formation of metaraorphic rocks and 

 of metalliferous deposits attested the power of the action of granite. 

 Besides this, Dufrenoy also found in the cretaceous formation the 

 traces of peculiar alterations due to the proximity of ophites. | 



Among the first observers of metamorphic phenomena it is just to 

 mention M. Keilhau, who in 1826 discovered that the deposit of 

 transition in the environs of Christiana is generally modified near 

 granite;§ and also Dr. Charles Jackson, who in 1827 announced that 

 amygdaloidal rocks were produced by the action of trap rocks on the 

 sandstone in Nova Scotia. 



CHAPTER IV. 



WORKS RELATING TO OTHER EFFECTS WHICH THE INTERIOR OF THE EARTH 

 EXERTS OVER THE EXTERIOR CRUST, AND WHICH ARE RELATED TO META- 

 MORPHISM. 



Besides the effects of transformation of which we have just spoken, 

 there are various other phenomena which appear also to result from the 

 action which the interior of our planet has exercised in ancient times, or 

 still exercises, upon the external crust; such are volcanoes, eruptions 

 of the older rocks, metalliferous deposits, and the mechanical disloca- 

 tions to which chains of mountains owe their origin. These different 

 chemical, physical, or mechanical effects of the internal action of the 

 globe are very probably associated, and it would leave a blank not 

 briefly to notice in this history the labors which have indirectly con- 

 tributed to illustrate the effects of metamorphism. 



§ 1. Eruptions of Volcanoes. 



The extinct volcanoes of central France, whose identity with ex- 

 isting volcanoesll was ascertained in 1751 by Guettard, revealed at a 

 later period to Dolemieu an important fact; it was that volcanic phe- 



"^- Especially in the environs of Saint Martin de Fenouillet. Caractferes de la craie dans 

 le sud de la France. — {Annales des Mines, 2d series, vol. viii, page 311, 1830.) 



f Memoir on the geological position of the principal iron mines of the eastern pirt of 

 the Pyrenees, accompanied by considerations on the time of upheaval of Canigou and on 

 the nature of the Rancy limestone. — [Anmves cles Mines, 3d series, vol. v, page 307, 1834.) 



:j: Memoir on the connexion between the ophites, the gypsums, and the salt springs of 

 the Pyrenees. — {Annales des Mines, 3d series, vol. ii, p. 21, 1832.) 



I'allaBsou also bad perceived a part of these modifications. — (See p 100 of the work cited 

 above.) 



§ M. Keilhau a.ssigned as a cause of these changes, which sometimes extend for 1,000 

 yards, simple molecular action, without the intervention of heat, nor of emanations from 

 below. He thought that eruptive were only sedimentary rocks situated at a gre.iter 

 depth, and become liquid. — [Darsielluiig der Uebergang's Formation in Norioegen, 1826; Ueber 

 die Bildung des Grimits. — Karstens Archiv., vol x, 1837. — Gwa Norvegica, 1, 1830.) 



[| Memoir on some of the mountains of France which have been volcanoes. — {M^moires de 

 ' Academie, 1752.) 



IG 



