E. E. L. Dixon — The Gavarnie Ocerthrust. 413 



with tliem to a slight extent. As seen at Heas gneisses and gneissose 

 pegmatites have been intruded into the schists, and when injected 

 lit par lit as in the case of No. 186 (p. 421, from Gavarnie), are often 

 difficult to distinguish from the intervening strips of partly digested 

 sediments on account of the paijallel arrangement of their own con- 

 stituents. When, however, they cross the banding of the schists their 

 igneous origin is obvious; their own foliation is then a continuation 

 of that in the adjacent schist, but is coarse and in'egular, being 

 frequently interspersed with ' augen.' A common type is a gneissose 

 biotite-granite, in which the biotite attains a large size (a length 

 of 2^ cms.); less commonly the gneiss contains black schorl along the 

 foliation planes, sometimes as ' augen.' 



Eoth schists and gneisses are seamed by later veins which are 

 distinguished by their unfoliated structure, and are themselves divisible 

 into at least two sets, the one being dark and possibly dioritic, and the 

 other, which in places crosses the former, consisting of white granites 

 and possibly pegmatites. 'Eo evidence was obtained of the time- 

 intervals between any of the intrusions, but no chilled margins were 

 observed. 



It is obvious, therefore, that there are important differences between 

 the Gavarnie and Bielsa granites. On the one hand, the latter forms 

 a coherent mass or boss of simple outline, almost if not entirely devoid 

 of veins of later granite, and surrounded by an aureole of normal 

 type, in Palaeozoic slates, in which the metamorphism, though intense 

 near the contact, diminishes steadily outward and shortly disappears 

 completely ; on the other hand, the Gavarnie mass is really a complex 

 in which (1) the commingling of igneous and sedimentary rocks is so 

 intimate that they are frequently difficult to distinguish from one 

 another in the field, and altogether beyond separation on a map ; 

 (2) the igneous constituents consist of several granitic injections, 

 including pegmatites, each represented by numerous veins, the later, 

 which are massive, cutting not only the sediments but also the earlier 

 veins, which themselves are foliated ; (3) no progressive metamorphism 

 is distinguishable in the sediments, which are, generally at least, 

 represented by crystalline schists and limestones.^ 



These differences are so great as to show that the Gavarnie granite 

 is wholly unconnected with the Bielsa granite. Consequently, as the 

 latter with a probable offshoot accounts for the belt of metamorphosed 

 Palaeozoics previouslj'- mentioned, there is no reason to regard the 

 proximity of this belt to the Gavarnie granite as other than accidental, 

 and therefore without any bearing on the question of the age of the 

 granite. 



The occurrence of chiastolite in the Silurian slates (Pig. 2) thrust 

 over the Cretaceous near Gavarnie is of little importance in view of our 

 ignorance of the original position of the altered rocks and the wide- 

 spread distribution of Hercynian granites capable of producing them. 



Bresson's next statement, however, that the Gavarnie granite 

 encloses not only mica-schists but also masses of unaltered ' schistes ' 

 and quartzites, would go far towards proving the Hercynian age 



1 Bresson states that unaltered sediments also are enclosed in the granite. 



