METAMORPHIC ROCKS IN FRANCE. 



great many places, and quartz and felspar bands occur in the granite. 

 In many places in the Pyrenees the " granite " contains beds of strati- 

 fied granular limestone such as in other districts lies in the gneiss, 

 with graphite, talc, fluor spar, mica, hornblende, &c. 



Boue, Dufrenoy, and other writers, have proved beyond a doubt 

 the powerful action of heat along the Pyrenean chain, as shown not 

 only by the usual subcrystalline character of the slates, but also by 

 the metamorphism of chalk into the condition of crystalline limestone, 

 with the development of abundance of metallic and granitic veins at 

 the line of junction of the altered, stratified, and igneous rocks. The 

 age of the eruption of granite along this chain is determined by 

 observations of Dufrenoy to be, at least in part, posterior to the Chalk. 



Igneous theory does not necessarily require that all these beds 

 of seeming granite should be altered gneiss, nor that the beds of por- 

 phyry should be considered as altered clay slate. Alternating igneous 

 and aqueous action is exemplified in modern operations of nature ; 

 but certainly in many cases, both in Cornwall and Cumbria, it ap- 

 pears the more correct view to suppose a gradual and partial rear- 

 rangement of the materials of the rock through the long-continued 

 action of heat. 



In Central France. The great central plateau of old rocks in 

 France, from which the Loire, Vieime, Dordogne, &c., take their 

 source, is chiefly a granitic and porphyritic tract, surrounded by oolitic 

 and carboniferous rocks, but slates and gneiss rocks appear in the 

 valley of the Yienne, and occupy a large part of the southern boundary. 

 Near Limoges are alternating beds of granite and gneiss, and some 

 subordinate beds of pegmatite and hornblende rock : the gneiss passes 

 by one variation to granite, by another to mica slate. The ranges of 

 strata near Limoges are north-east and south-west, and they are 

 crossed by decomposing elvan courses to north-north-east. Tin veins 

 occur near Yaulry in gneiss as well as in granite. Towards the 

 borders of the district the gneiss becomes less granitic, more associated 

 with hornblende slate, and encloses deposits of micaceous limestone. 

 Serpentine lies in this gneiss in many places. The pegmatites and 

 kaolins of St. Yrieux, which have resulted from decomposition of 

 this rock, form numerous veins and strings in the gneiss and horn- 

 blende slates, whicli sometimes intercalate themselves between the 

 laminse. Quartz rock of bluish colour exists in the Black Mountain 

 and elsewhere. Oxide of iron abounds at many points in the gneiss ; 

 galena, phosphate of lead, carbonate of copper, antimony, and haematite 

 are the products of the veins. 



The most remarkable alterations of secondary limestones take 

 place, according to Dufrenoy, along the line of junction with the 

 granitic and porphyritic masses. Thus the Lias and Oolite become 

 metamorphic, and are traversed by metalliferous veins, just as the 

 slates in Cornwall and Brittany are metalliferous, principally in the 

 same situation. 



Other Localities in Europe. After these details of the circum- 

 stances attendant on gneiss and mica slate at so many interesting 



