354 Dr. Dw Riche Preller—Crystalline Rocks, N. Piémont. 
5. The formation of different groups and combinations of basic and 
acid rocks out of the submarine, viscid, heterogeneous magma slowly 
welling up from the reservoirs of molten material in the centres of 
eruption, took place according to chemical and mineralogical affinities, 
the basic material separating out, grouping, and crystallizing more 
rapidly, the acid more slowly. Hence the basic rocks of the Ivrea 
belt were probably consolidated at a somewhat earlier stage than the 
acid rocks, but within the same geological period. 
6. The transformation of the pietre verdi with primitive to those 
with secondary elements is the combined result of metamorphism 
and of decomposition by hydration which led to the modification and 
re-formation of the decomposed elements to secondary groups and 
combinations. This process is much less in evidence in the pietre 
verdi of the older or mica-schist than in those of the younger or 
calc-schist formation, the mica-schist being much less permeable and 
magnesian than the cale-schist. The transformation probably took 
place in many cases during the initial stage of consolidation after the 
eruption of a highly magnesian magma in the cale-schist formation, 
e.g. in the areas of Monte Viso and the western part of the Lanzo 
valleys. 
7. Chemical and mineralogical affinities produced in some cases 
basic rocks which, though frequently associated, preserved their 
separate entities, e.g. the associated but separate belts—originally 
parallel lava streams—of diorite and porphyrite (melaphyre) of the 
Ivrea area; or again, peridotite and gabbro and their derivatives 
serpentine and euphodite. The frequent veins and dykes of gabbric 
in peridotitic rocks show that the former compenetrated the latter 
when these, as the more basic, were already in course of submarine © 
consolidation or, alteration. Secondary amphibolites and prasinites 
are derived not only from diorite and diabase, but very largely, 
by metamorphism, from gabbro and euphodite which, in common 
with the other pietre verdi, also decompose readily to serpentinous 
schist or pseudo-serpentine. The gneissiform, calciferous, and 
schistose pietre verdi are the result, in some cases, of pressure- 
metamorphism, e.g. gneissic gabbro, gneissic diorite, etc.; in others 
of compenetration and metamorphism of sedimentary and eruptive 
material from tuffs and muds, e.g. diabasic and porphyric schists, 
or in relation to calciferous rocks, amphibolic and pyroxenic 
‘‘calcefiri’? and serpentinous ‘‘ ophicalce”’. 
8. The principal earth-movements experienced by the Piémontese 
Alps after their emergence came from the south-east, viz. from the 
Mediterranean and the Po Valley, as is shown by the precipitous 
and greatly folded flanks of the Maritime and Eastern Cottian and 
Grajan Alps which, together with the Lanzo—Ivrea—Val Sesia area, 
formed a coastal region contiguous to the Po Valley. Another, 
intermediate movement proceeded from the west and corrugated 
the great synclinal cale-schist formation. The principal fracture- 
faults of the Piémontese Alps are: (a) in the south, the transverse 
faults of the Stura di Cuneo and of the Col di Tenda and 
Vermenagna Valleys, along the north-western and eastern bases of 
the Argentera massif; (6) in the north, the transverse fault along _ 
