Vol. 50.] GNEISSES IN THE COTTIAN SEdUENCE. 273 



schists and the igneous rocks of the ' pietre-verdi ' group have been 

 foliated at right angles to the axis of compression. Though I am 

 not aware of any evidence that gives positive proof of the exact age 

 of these dislocations, it is highly probable that the} 7 are part of the 

 great post-Helvetian pre-Tortouian movements in which the forces 

 that raised the Western Alps attained their maximum intensity. 



The Waldensian gneisses have not been affected by any of the 

 previous movements. The differences in this respect between the 

 gneisses and the neighbouring schists is one of the most striking 

 features in the Cottians. In the gneisses the minerals are all fresh, 

 and the original fluxion-foliation undisturbed. In the schists the 

 minerals are altered, often till no trace of the original constituents 

 remains ; instead of the felspars being water-clear, the original 

 fragments are saussuritized or represented only by grains of zoisite, 

 while the pyroxenes have been uralitizcd, and the resulting pseudo- 

 morphs, as well as the original amphiboles, have been converted 

 into smaragdite. The changes in the structures and field relations 

 of the rocks have been still more striking ; the schists have been 

 intensely crumpled and contorted, folds have been inverted, and the 

 materials of the concave limb crushed into mylonites ; gnarled 

 schists have been a second time crumpled, foliation has been im- 

 pressed on foliation, and fault has broken the continuity of fault. 

 It seems almost impossible to believe that the gneisses can have 

 remained fresh and undisturbed through the dislocations that have 

 produced such changes in the rocks with which they are in contact ; 

 this affords another proof not only that the Waldensian gneisses are 

 younger than the schists, but that they are later than the great 

 earth-movements at the close of the Middle Miocene. It is quite 

 possible that the gneiss of the enormous massif of the Paradiso was 

 intruded somewhat earlier than that of the Eastern Cottians, and 

 this would account for the pebbles in the Miocene. The low banks 

 of gneiss which even now have only a very restricted outcrop, some- 

 times merely on the floors of the Waldensian valleys, are most unlikely 

 to have been exposed during Miocene times. 



In the Alps of Dauphine the Pliocene movements are very feeble, 

 but this does not forbid their powerful influence in the centre of 

 the Cottian group, for feeble also are the movements in the Miocene 

 there — compared with those which elevated the main chain of the 

 Western Cottians. The elevation of the Villafranchian beds has 

 been shown by Sacco to demonstrate an elevation of over 1500 feet, 

 and the gneiss-intrusion may well have been contemporary with 

 this. Paradoxical though it may appear, the evidence renders it 

 most probable that the Waldensian gneiss, instead of being of 

 Laurentian age, is really Pliocene, and, with the exception of 

 the Saharian and recent alluvium and the glacial moraines, is the 

 newest rock in the Cottians. 



