270 EXPERIMENTS ON METAMORPHISM AND 



of lime has disappeared. Indeed, we notice that pure limestone never 

 alternates with them. 



Stratified dolomites are in general associated with deposits of an- 

 hydrite and gypsum, rocks which have been also considered as owing 

 their origin to an epigeny ;* they likewise frequently accompany de- 

 posits of rock-salt. These last three rocks, which have remarkable 

 analogies in their modes of occurrence, have been, like dolomites, 

 referred to two types of formation. Most frequently they are regu- 

 larly subordinated to stratified formations, of which they constitute 

 a characteristic element, as we see in the trias of western Europe 

 and of Spain, and in the tertiary formations of the Carpathian moun- 

 tains. Elsewhere there are casual deposits w4iich appear to be di- 

 rectly connected with dislocations, as is admitted in regard to the 

 beda of the Alps of Salzbourg and of Bavaria, of the Pyrenees, and of 

 Algeria ; thus the characteristic of a double origin attributed to dolo- 

 mites is also found in the gypsums, anhydrites, and rock-salt which 

 accompany them. 



A very remarkable character of all formations containing salt, and 

 which also shows the analogies and the singularity of their formation, 

 is the uniform or variegated red tint of the clays, sandstones, and even 

 of certain masses of the salt which compose them. I shall not be 

 able here to reproduce the considerations by Avhich Elie de Beaumont 

 has compared these different connected facts, and has shown that in 

 the mass of waters, which deposited the marnes msees, phenomena 

 analagous in their results to those Avhich accompany volcanic action 

 have taken place, t These red formations often cover perfectly regu- 

 lar beds, the coloration of which presents nothing abnormal; such are 

 the marries irisees which rest upon the muschelkalk. This circum- 

 stance, in connexion with the vast superficies which they often oc- 

 cupy, would seem to show that the heat of the earth had only exercised 

 its action upon them in an indirect or circuitous manner, by heating the 

 water of the sea. It would, therefore, perhaps be in virtue of a high 

 heat and a chemical action with which the concentration of chloride of 

 sodium was not unconnected, that the sea itsel-f tinted the saliferous 

 formations, thus leaving, as it were, a witness of a temperature which 

 it had exceptionally experienced at certain epochs and in extensive 

 portions of its basin. 



We have just said that the regularly stratified dolomites are at- 

 tributed to an epigeny of limestone, and that it was at the time 

 of the deposit of this limestone and before it was protected by other 

 depositions that this transformation had taken place; now, precisely 



* Explication dt la Carte Giologique de France, vol. ii, p. 90. 



f Explication de la Carte Geologique de France, vol. ii, p. 94. 



De yenarmont lias discovered that peroxide of iron may be dishydrated even ia 

 water, at a temperature betvveeu 160'^ and 180° centigrade. Tliis reduction, in an experi- 

 ment that 1 made, took place in a saturated solution of chloride of sodium at only 150°. 



I also verified, by analysis, that the red part of the variegated clays not only differed 

 from the green parts by the state of combination of the iron, but also by a very much 

 greater proportion of the metal. This observation led me to imitate exactly the ordinary 

 variegations of clays in passing successively vapors of chlorhydric acid and water over hot 

 clay. Some parts of the clay became colorless, while other parts took a red tint at the 

 expenee of the first. 



