ON THE FORMATION OF CRYSTALLINE ROCKS. 251 



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

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

 been at the very time of its deposit. This reservation lias 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 will mention only 

 the observations of Hoffmann and of Coqu'and on the gypsums 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.f 



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 different 

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

 Voigt,§ Mohs, II de Hoff",! and Schmidt^* ought to be cited. In Eng- 

 land, John Phillips had clearly recognized the difference between 

 the true and the /aZse 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 w^as 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 diff^rents formations qui, dans le system des Vosges, g^parent 

 la formation nouill6re de celle du lias. — [Mimoires pour servir a la description geohgique de la 

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



t Among the principal I mention the work of d'Alberti, the memoirs of Fournet and 

 of Boue — (Ueber die Dolomite. Sitzungs Berichte der Acml. der Wifnens. zu Wim, vol. xii, p. 

 422; 1854.) 



I Das Ilarzgehirge. 



^Praktische Gehirgshmde, 1707. 



\\\Molls Ephemerifkn, vol. iii, p. 71 ; 1807. 



•[j Slate quarries of Lehesten, in Franconia. 

 «c- In Westphalia. 



If On the chemical changes produced on the aggregate of stratified rocks. — {Tramadions of 

 the Gc'ilogical Society, vol. iii, p. 354 ; 1835 ) 



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

 same distinction in the slates of the Ardennes. — (Explicaticm de la carte gcologique de France, 

 vol. i, p. 262.) 



