592 METAMORPHISM IN THE PYRENEES. 



five distinct alternations of granitic beds, with as many layers of mica 

 slate, are clearly traced, and several of these beds are made up of 

 similar alternations of granite and mica slate or quartz and mica slate. 

 The great mass of granite is below, and the great mass of mica slate 

 above, constituting the hill called Lugduff. Granitite abounds in this 

 slate. 



Similar alternations occur in other neighbouring places, making a 

 total thickness of one-third of a mile, and the whole system ranges 

 north-east and south-west, and dips south-east. On the north-east 

 they probably abut and terminate against the granite. The mica 

 slate on the summit of Lugnaquilla is likewise interstratified with 

 granite. Clay slate bounds it on the east, and at length coming into 

 contact with the granite, cuts off its farther progress to the south. 



On the western side of the granite the mica slate is still less ex- 

 tensive. It is found to enclose beds and elliptical masses of granite 

 in Glenismaule ; and it is mentioned that a granite vein, four to eight 

 inches wide, ranging 25 north of west, cuts off the mass of alternating 

 strata, without occasioning any displacement. In the same valley are 

 two distinct beds of compact doleritic rock in the mica slate, one 

 four feet wide, the other two feet. Andalusite abounds in the mica 

 slate of this country ; and dolerites alternate with it. 



The frequency of the phenomenon of alternation between mica 

 slate and granite is a feature in the geology of this part of Ireland 

 long since discussed by Mr. Weaver. 1 



In Brittany. The tract of oH rocks in the north-western part of 

 France is one of the most extensive in Europe. The granite, gene- 

 rally the most elevated, is separated from the secondary strata by 

 gneiss and mica slate, and lower palaeozoic slates, into which they 

 pass almost indefinitely. In the departments of Calvados and La 

 Manche these two systems appear as zones around the granite, the 

 gneiss being within the clay slate. Quartz rocks of blue colour, and 

 pegmatites with tourmaline, are associated with them ; and they are 

 traversed by veins of quartz and granite. 2 



In the Pyrenees. The granitic masses of the narrow chain of the 

 Pyrenees having been uplifted in much confusion, are very irregularly 

 bordered ; in several places they are overlaid by gneiss and mica 

 slate, but generally by the latter series. Charpentier regarded the 

 gneiss of the mountains which border the valley of Soulan as inti- 

 mately connected by gradation and alternation with the subjacent 

 granite, so as to be necessarily united therewith into one formation. 

 In many instances gneiss and granite are described as alternating in 

 very thin layers. In other cases, vast blocks of micaceous gneiss of 

 100 cubic fathoms bulk are buried at intervals in granite, always 

 preserving one constant relative position or direction of the lamina. 

 These are thought by Charpentier to be of contemporaneous origin 

 with the granite, which passes into them at the sides, and thus inter- 

 laminates the gneiss. 



Mica slate, in the same manner, is intercalated with granite in a 



1 Geol. Trans., vol. v. 2 De Cavnnont, Geol. du Cavaldos. 



