ON THE FORMATION OF CRYSTALLINE ROCKS. 251 



' In many stratified formations, however, such as the trias, dolomite 

 is found in regular beds ; if there has been a substitution it must have 

 been at the very time of its deposit. This reservation has been made 

 by Elie de Beaumont, especially in favor of the dolomites of the 

 Marnes Irisees.* 



It has been also discovered that in certain formations the gypsum 

 resulted from a transformation of the limestone. I Avill mention only 

 the observations of Hoffmann and of Coquand on the g3'psums produced 

 by the fumaroles of the Lipari islands and in Tuscany, and those of 

 Dufrenoy on the gypsums associated with the ophites of the Pyrenees. 



The formation of the dolomite, of anhydrite, and of rock salt, has 

 given occasion to very numerous works, which it is impossible to men- 

 tion here.t 



CHAPTER VI 



METAMORPHISM OF STRUCTURE. 



In the time of Hutton it had not been remarked that the foliated 

 structure so frequent throughout entire masses of rocks, results from 

 an action posterior to their formation, and consequently thus consti- 

 tutes metamorphism. 



The distinction which we now recognize between a foliated struc- 

 ture and stratification had, however, been described by difi"erent 

 authors at the commencement of this century, among whom, Lasius, | 

 Yoigt,§ Mohs, II de HofF,T[ and Schmidt*'^ ought to be cited. In Eng- 

 land, John Phillips had clearly recognized the difference between 

 the true and the/aZ.se cleavage. Professor Sedgwickff afterwards con- 

 firmed and generalized the fact, showing that in Wales the plates are 

 very generally oblique to the stratification. |J 



Another very remarkable circumstance, which was signalized by 

 Mr. Sedgwick, is the astonishing constancy with which the plates con- 

 form to one another over extensive tracts, and even amidst the most 

 marked convolutions of the beds to which they belong. This ob- 

 servation has been completely confirmed by Studer and Forbes in 



* Observations sur les diffe'rents formations qui, dans le system des Vosges, geparent 

 la formation nouill^re de celle du lias. — {3f^moires pour serrir a. la description geologique de la 

 France, vol. i, pp. 78, 153, 192.) 



f Among the principal I mention the work of d'Alberti, the memoirs of Fournet and 

 of Bone. — {Ueher die Dolomite. Sitzungs Berichte der Acad, der Wissens. zu Wien, vol. xii, p. 

 422 ; 1854.) 



J Das Harzgehirge. 



§ Praktische Gebirgskunde, 1797. 



\\1MoUs Ephemeriden, vol. iii, p. 71 ; 1807. 



•[f Slate quarries of Lehesten, in Franconia. 

 ** In Westphalia. 



ft On the chemical changes produced on the aggregate of stratified rocks. — (Transactions of 

 the Geological Societg, vol. iii, p. 354 ; l.?35 ) 



XX Mr. Parrot, engineer of mines, according to a manuscript report of 1826, noticed the 

 same distinction in the slates of the Ardennes. — (Explication de la carte geologique de France, 

 vol. i, p. 2e2.) 



