262 EXPERIMENTS ON METAMORPHISM AND 



and particularly when the eruptive rock is of a granitic nature, the 

 extent of the modified zone, as well as the more complete changes 

 that have taken place, denote a much more energetic action. 



Not only the extent of the modified zone varies according to the 

 nature of the eruptive rock, but for the same rock, and in the same 

 country, this extent presents great difi"erences.* Near the granite 

 it is often of some hundred yards, and, exceptionally, even 3,000 

 yards: for instance, in the environs of Christiania this band is on an 

 average 360 yards; in the Pyrenees it attains 1,500 yards, perfectly 

 characterized in its effects, t In general, it is remarked that the 

 transformation has extended further between the re-entering angles 

 formed by the eruptive rock than opposite the salient parts. — (Champ- 

 du-Feu in the Vosges,:}: environs of Christiania.) With regard to the 

 nature of the modifications undergone by the surrounding rocks, they 

 are so various that it is difficult to give a summary of them.§ 



Sometimes only a new molecular arrangement has taken place; lime- 

 stone has become saccharoidal like statuary marble; elsewhere sand- 

 stone has been changed into quartzite. — (Isle of Sky.) The mineral 

 combustibles have generally been modified by losing a part of their 

 constitutive elements. || It is thus that lignite has been changed into 

 coal, into anthracite, and sometimes even into graphite, (graphite 

 mined in the tertiary formation at Omenatk, in Greenland, in which 

 formation it is also known in Java.) Coal has sometimes been changed 

 into one of these two last states, (graphite of Scotland, graphite and 

 anthracite of Worcester, Massachusetts.) More rarely coal and lig- 

 nite have been transformed into a kind of coke. IF Bitumen, accidentally 

 isolated from these combustibles, has been lodged in the rocks which 

 were more or less near. — (Lobsann, in the Bas-Rhine, Hering, in Tyrol.) 

 Most frequently new crystalline combinations have been developed, 

 either with the pre-existing elements of the rock, or by the aid of 

 new elements which have been introduced, or by the elimination of 

 some of those which were originally there. *^ 



™ The chalk of the northwest of Ireland, near certain veins of trap, is not modified in the 

 least ; it has, on the contrary, become crystalline near those that are very large ; in this 

 last case the modification rarely extends beyond three yards. The same rock forms veins in 

 the Isle of Sky, in Scotland ; near some of them the lias is modified, whilst it is not at all 

 affected near others, nor are we able to account for the cause of this difiference. — 

 {Karsten's Archiv., vol. 1, 2d series, p. 99.) The rocks of the transition formations of the 

 Vosges, into which granite has penetrated in veins, presents still greater diffei'ences ; 

 sometimes the modification is imperceptible, as in the valley of Wesserling ; sometimes it 

 is very distinctly marked, as at Andlau and at Barr. 



f Durocher, memoir cited above. 



j The transition formation is modified in a manner much more complete and over a 

 greater expanse at the top of the valley of Barr than in the valleys of Ville and d'Andlau; 

 and this appears to result from the fact that, instead of simply bordering on the granite, it 

 forms in the first locality a long band, which is as it were let in between the granite and 

 the syenite. — {Description Geologique du Bus Rhin, p. 54.) 



^Annales cles Mines, 5th series, vol. xii, p. 89. 



II Which does not prevent them from, often having acquired new minerals like the other 

 rocks, as the zeolitlis, for instance. 



<[ This last transformation, pointed out near Newcastle, for instance, has not as yet been 

 observed in the proximity of granitic rocks. 



*■''" It is this last that appears to have happened in the granxdar quartz of Brazil. 



