264 EXPERIMENTS ON METAMORPHISM AND 



many it there bears the name of schaalstein.* We know, moreover, 

 that the eruptive rock itself often undergoes modifications in the 

 neighborhood of the enclosing rocks. f 



The different transformations which I have been noticing form, so 

 to speak, irregular aureoles around granite and the other eruptive 

 rocks. Elie de Beaumont has shown that, according as the rock 

 is acid, that is, contains an excess of silicic acid or basic, the metal- 

 liferous deposits in connexion with it present two distinct types. 

 The same is true of the metamorphic aureoles of which I have just 

 spoken, and the observations of Delesse have contributed to prove 

 it. Thus, on the one hand, the zeolites which have so often been 

 found near trap rocks have not been found near effusions of granite. :j: 

 On the other hand, this last rock produces certain minerals, to the 

 exclusion of all others, such as, for instance, the aluminous silicates, 

 known under the name of made and staurotide, so common in the 

 argillaceous schists of Britanny. Micaceous and feldspathic schists 

 very frequently envelop granite to a great depth in the Pyrenees and 

 elsewhere ; we do not know of anything analogous near the traps. 



CHAPTER II. 

 REGIONAL METAMORPHISM. § 



I repeat that I am speaking here of the schistose masses whose meta- 

 morphic origin is clearly proved; consequently I refer to the third part 

 of this memoir, the examination of primitive gneiss, mica schists, and 

 other subordinate rocks, which are inferior to the stratified fossiliferous 

 formations. Considerable masses of stratified rocks, occupying entire 

 counties, often exhibit very marked traces of metamorphism, even 

 when it is impossible in these formations to discover the least out- 

 crop of eruptive rocks. II This modification is easily verified in coun- 

 tries where it has been of so feeble an intensity as not to have entirely 

 destroyed the sedimentary character of the rock ; such are Wales, 

 the Taunas, and the Ardennes. In the silurian and devonian periods 

 of this last locality the rocks have in part become schistose, and over 



« These amygdaloidal rocks frequently pass to fossiliferous limestones, and often them- 

 selves take the form of a conglomerate. (Steingrahen, in the Vosges, Nassau, county of 

 Brilon in Westphalia, where they are associated with a Labradoric porphyry ; Paimpol, in 

 Britany, Lake Superior, and Nova Scotia ) Certain spilites of the Alps and of I'Esterel are 

 considered as metamorphic by Mr Gras. 



f W^hence the name of endovwrphism, proposed by Fournet ; often, according to Delesse, 

 it is impregnated with a magnesian hydrosilicate. 



X Durocher ; memoir cited above, p. 607 and 614. 



§ The name of regional metamorphism, which I have proposed, appears to me more 

 exact than normal metamorphism, and less vague than that of ^general metamorphism. 



II The difference in the nature of mineral combustibles, lignite, coal, anthracite, which 

 varies according to the formation, may be considered as an example of a metamorphi.sm 

 that has taken place without the intervention of eruptive rocks ; it has acted, perhaps, on 

 more impressionable substances than rocks. . It is on this account that anthracite only is 

 found in the Alps and in the talcose schists of the Basse-Loire, and that the eocene forma- 

 tion of Tuscany contains a real coal. — (3Iontu Bamholi.) 



