and other Problems in Pyrenean Geology. 367 



of Cretaceous limestones and, in some places, the Palaeozoics immedi- 

 ately above, retain, mtra se, their original order of superposition, 

 and have not been reversed, thus putting a recumbent fold out of 

 court. Also the junction itself bears witness to a lateral thrust of 

 the most powerful kind. This is clearly the case in the cirque of 

 Troumouse at the exposure previously mentioned. The chiastolite- 

 slate has suffered most, as it has been ' pugged up ' or mylonised for 

 some distance above the junction, but, though it has proved such an 

 excellent lubricant that undistorted hippurites may be found in the 

 limestone a short distance below, nevertheless the latter has also 

 had a rough time. This is evident from the occurrence in the 

 ' pugged up' slate of large and small ' augen' (shown in PI. XVIII, 

 Fig. 2) of limestone torn from the sheet below, and from the presence 

 of shear-zones in the limestone itself, even down to its base. Along 

 these zones the limestone has been reduced to a fine-grained, platy 

 mylonite. Unfortunately on the occasion of our visit slickensiding of 

 the upper surface of the limestone was not accessible and the direction 

 of movement of the slates, therefore, not directly determinable, on 

 account of weathering along the junction. 



Finally, Bresson points out that the overthrust Palaeozoics must 

 have moved in a southerly direction ; this is obvious from the fact 

 that there is, or has recentty been, an unbroken mantle of younger 

 rocks for a great distance to the south, whereas to the north, in which 

 direction the thrust-plane is inclined. Palaeozoic rocks, similar in 

 facies to and continuous with the overthrust mass, extend far beyond 

 Gedre and the vallej* of Saux, where the underlying Cretaceous 

 limestone disappears, and with it evidence of the movement. In 

 fact, as the rocks and structures above the exposed portion of the 

 thrust-plane are similar to those of the disturbed zone to the north, 

 it is difficult, if not impossible, to say where the overthrust begins or 

 by how much its full width may exceed the 10 kilometres which, as we 

 hare seen, is exposed. The view that it came from the north is fully 

 borne out by the inclination of the folds (Fig. 1) in both the overthrust 

 rocks themselves and the Cretaceous mass piled up in front. The folding 

 of the latter is remarkable, and is beautifully figured hj Carez. That 

 the southward movement encountered great resistance is suggested by 

 this folding and confirmed by the remarkable form of the front of the 

 thrust-plane, which instead of sloping steadily upward ends southward 

 as a steep fault with an irregular east-and-west course, and, in places, 

 with possibly even a southerly hade like a normal fault.' 



The evidence which has been cited amply justifies, I think, Bresson's 

 charriage. In fact, although it forms but a part of what has been 

 worked out with much care by Bresson himself, it would not have 

 been given at such length were it not that the truth of parts of it 

 have been questioned and, more particularly, that another and much 

 larger overthrust has been stated to exist, in the same region but on 

 the strength of evidence Avhich, in contrast to that of Bresson, appears 

 to me so slight as not to justify such sweeping conclusions. 



The discussion of this second problem has been prompted by some 



1 Bresson, op. cit., fig. 75. 



