GEOLOGY OF THE WESTERN ALPS. 165 



lying schists, which have been caught up and altered at 

 the time of the intrusion of the gneiss. In many places 

 the gneiss at the margin is saturated with chloride and 

 amphibolic material caught up from the schists by the 

 intrusive rock. In addition to this " endomorphic " altera- 

 tion of the gneiss, there is a well-marked " exomorphic " 

 alteration produced in the beds into which it has been, 

 intruded ; among others the following minerals have been 

 developed by this contact metamorphism : quartz, white 

 mica, biotite, orthoclase, microcline, oligoclase, garnets, 

 kyanite, epidote, zoisite and sphene. The extent to which 

 the beds have been altered varies enormously ; sometimes, 

 when the intrusive rock was small in bulk, it has only 

 affected a few inches on either side ; at other times the 

 rocks have been intensely altered for fifty feet from the 

 contact. A second line of evidence is offered by the 

 occurrence of apophyses or dykes which run from the main 

 mass of gneiss into the schists. The best case described 

 occurs in the Angrogna Valley, a few miles from the town 

 of Torre Pellice ; it is illustrated by a photograph which 

 shows the white veins of the aplite, ramifying through 

 darker - coloured beds belonging to the schist series. 

 Similar cases of the same sort occur in the Pellice Valley, 

 in the Valley of the Po near Crissolo, etc. A third proof 

 of the intrusive character of the schists is afforded by the 

 mapping of the contact lines, for there is a transgressive 

 junction between the gneisses and the schists. 



Another very striking- point is the fact that none of the 

 old igneous rocks that cut through the schists enter the 

 eneisses. Both the calc-schists and mica- schists contain 

 many beds of greenstone, which no doubt represent altered 

 igneous rocks, some of which were certainly intrusive. 

 These occur in such abundance in the schists that these 

 are called the " pietre verdi " series. 1 These greenstones 

 are amphibolites and serpentines, and they run through the 



1 It should be remembered that the name "pietre verdi" is used not 

 only for the greenstones themselves, but also for the series of schists in 

 which these rocks are most abundant. 



