162 Dr. C. S. Du Riche Preller—Pretre Verdt. 
stage of alteration, and still more from serpentinous schist, which, 
occurring frequently as intermediate between crystalline schists or, 
again, between crystalline limestone and pietre verdi, is the product 
of chloritic decomposition of the latter. As such it may, by the 
abstraction of magnesia, be derived from any of the basic rocks with 
altered elements, notably from euphodite, amphibolite, and prasinite, 
or their schists, although the prototypes of these rocks—gabbro, 
diorite, and diabase—hayve, apparently, no identity of origin or affinity 
with serpentine proper. So-called serpentinous schist is therefore 
pseudo-serpentine, and represents, together with chloritic and talcose 
schist, probably the last stage of alteration and metamorphism, not 
only of serpentine but of some of the other pietre verdi series, 
The rocks with altered elements often assume a laminated, gneissi- 
' form structure, and exhibit a marked affinity with gneiss or, 
again, with mica-schists and even with calc-schists. Thus minute 
eneiss becomes amphibolic, prasinitic, or epidotic ; mica-schist 
becomes epidotic and even more frequently glaucophanic when in 
association with the blue glaucophane or gastaldite variety; while 
ovardite, when very rich in chlorite and taking up mica, quartz, and 
calcite, passes into prasinitic cale-schist and phyllite. 
The massive eruptive rocks with primitive elements—diorite, 
diabase, gabbro, pyroxenic-biotitic porphyrite (Gastaldi’s melaphyre), 
and the enormous peridotitic masses—occur more especially in the 
gneiss and mica-schist area of Northern Piémont, that is, in the 
so-called dioritic belt or ‘Ivrea zone’ which extends from the eastern 
spurs of the valleys converging near Avigliana, west of Turin, to the 
Lanzo spurs, and thence north-east to Ivrea, Biella, and the Sesia 
valley, and beyond the latter to the Strona valley near Lake Orta. 
The same ‘Ivrea zone’ also extends into the Aosta valley and the 
valleys descending from Monte Rosa. The amphibolic, prasinitic, 
euphoditic, and serpentinous series with secondary elements, on the 
other hand, predominate in the cale-schist and mica-schist area from 
the upper Lanzo valleys south to Monte Viso and the Maritime Alps, 
and extend into the Permian and Triassic formations of the latter. 
The considerable euphoditic and diabasic masses of the Grana and 
Maira valleys south of Monte Viso are all more or less profoundly 
metamorphosed to epidotic (zoisitic), amphibolic, and prasinitic rocks,* 
and therefore do not belong to the category of eruptive rocks with. 
primitive elements. 
The constant and intimate association of the pietre verdi not only 
with each other but with the stratified mica- and cale-schists led 
Gastaldi, as previously stated, to regard all those rocks, with the 
only exception of the primary eruptive rocks of the Ivrea belt, 
indiscriminately as metamorphosed sedimentary.?, The pietre verdi 
are now generally recognized to be in the main derived from 
eruptive rocks and some probably from tuffs or muds. At the same 
time the constant alternations, amounting to interstratification, of the 
1 §. Franchi, ‘‘ Aleune metamorfosi di eufotidi e diabasi Alpi Occid.’’: Boll. 
R. Com. geol., 1895, p. 181 et seq. 
2 Hence his well-known dictum: ‘‘In the Piémontese Alps plutonism is. 
a myth.”’ 
