HEM1THRENES AND OTHER CALCITIC ROCKS. 51 



calcite occurs in the dyke may be taken as positive evidence 

 that this mass has also to some extent undergone a chemical 

 transmutation. In short, we feel it a safe conclusion that the 

 change in both rocks is due to their having been penetrated by 

 heated water containing a carbonate or carbonic acid in solution*. 



In addition to the important evidence which the hemithrene 

 of Mont Saint-Philippe has afforded in connexion with the 

 chemical changes effected in minerals, we have next to make 

 known some facts from the same place which bear directly on 

 the matter under consideration. 



The region of the Vosges, which embraces Wisembach, Chippal 

 near Croix-aux-Mines, Laveline, Gemaingoutte, and Mt. St.- 

 Philippe near Sainte Marie-aux-Mines (Haut-Rhin), consists 

 principally of gneiss, with here and there intrusive masses of 

 igneous rocks (syenites and dolerites) ; but in the places named 

 there is a development of hemithrene. 



A large quarry of this rock (" calcaire saccharo'ide") is worked 

 at St. -Philippe. It is a nearly rectangular excavation, about 60 

 or 70 paces long, 30 paces wide, and 15 feet in depth. The rock, 

 of a highly crystalline character, is for the most part well strati- 

 fied, as is also the associated gneiss : both dip in the same direc- 

 tion (south-east), at an angle of about 20. The hemithrene, 

 light in colour, is more or less charged with pyrosclerite, chiefly 

 in granules ; and the same mineral occurs, but rarely, in the 

 immediately adjacent portions of the gneiss. The pyrosclerite 

 and other mineral silicates, as previously notified, are irregularly 

 intermixed with calcite ; but very often the different kinds (car- 

 bonates and silicates) are disposed in layers : on account of the 

 latter arrangement the beds have assumed a laminated structure. 

 The lamination is often irregular, being variously undulated, and 

 separating here and there through the interposition of com- 

 pressed lumps which consist internally of a white granular or 



* Quart. Jourii. Geol. Soc. vol. xvii. p. Iviii. A short time ago Brcegger 

 and Reusch observed at Hiasen, in Norway, " a vein of hornblende changed 

 into a mass of calcspar" (Canadian Naturalist, n. a. vol. viii. p. 430). Both 

 this case and the calcified doleritic dyke at Cleggan may be taken to prove 

 that the well-known calcitic dyke, traversing nietaniorphic rocks near Auer- 

 bach in Bergstrasse, first described by Von GEynhausen more than fifty years 

 ago, was originally a silacid igneous mass, and since converted into hemi- 

 threne : its accessory minerals (hornblende, treniolite, idocrase, wollastonite, 

 epidote, and the like) favour this view. 



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