364 



E. E. L. Dixon — The Gavaniie Overthrmt, 



from the path leading to the Hourquette d' Alans by an observer 

 looking westward toward Yignemale (3,298 m.), which, with its 

 glaciers, appears in the distance. The lower declivities of the 

 valley, below the gentle slopes stretching into the middle distance, 

 form a deep, steep-sided trench in the crystalline rocks of the basal 

 platform. The western (further) side of this trench is seen as a steep 

 slope streaked by light-coloured talus. At the top the thin sheet of 

 Cretaceous limestones resting on the platform appears in the middle of 

 the view as a dark, discontinuous band, falling gently northward (see 

 also Fig. 3). It turns into the side valley, the valley of Ossoue, 

 which di'ains the slopes of Yignemale itself, and on the northern side 

 it is conspicuous on the ground, though barely distinguishable in the 

 figure, beneath two dark bands in the lower slopes of the Pic de 

 Culaous. Outside the view it swings round again and continues down 

 the main valley. On the eastern side it is not seen in the figure as it 

 crops out along the brink of the deep trench. Above it the contours 

 on both sides are more irregular, the Paloeozoic rocks which give rise 

 to them exhibiting great variety of hardness and structure. Thus the 

 Pics de Mourgat and Lapahule seen immediately above the western 

 side of the main valley, have been found by the French geologists to 

 be composed respectively of Middle Devonian limestone and Carboni- 

 ferous limestones, etc., separated by Lower Devonian slates across the 

 intervening col. 



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Fig. 3. — Structure of the lower part of the Pic de Mourgat. The summit 

 is hidden. (From a photograph.) 



The structure, according to their mapping, of the lower part of 

 the Pic de Mourgat, as well as the outcrops of the Cretaceous and 

 crystalline rocks below, is more clearly seen in Fig. 3, drawn from 

 a photograph. In PL XVIII, Fig. 1, the dark bands in the northern 



