160 Dr. C. 8S. Du Riche Preller—Pietre Verdi. 
The new classification, besides harmonizing with the most recent 
interpretations west and north of the Alps, has the signal advantage 
of having eliminated Gastaldi’s ‘‘ pietre verdi zone”, which, in his 
separate and far too comprehensive sense, had become a fruitful 
source of misconception. As experience has shown, pietre verdi are 
not peculiar to any particular horizon, and throughout Italy, as 
elsewhere, occur in all the formations from the Hocene down to the 
Paleozoic; in the Piémontese Alps, though in a special form, even 
in the primitive gneiss. 
II. CLAssIFICATION OF THE PIETRE VERDI. 
In order to avoid tedious repetition, it will be convenient to 
specify briefly the leading varieties of the pietre verdi, some of 
which have characteristics peculiar to the Piémontese Alps. Gastaldi, 
with his wonderful intuition and perspicacity, laid down certain 
broad lithological distinctions which, in the main, are still correct. 
They were used by hisimmediate contemporary followers Striiver and 
Baretti, and after them by Bucca,’ in their excellent and diffuse 
macroscopic and microscopic investigations, and until recently also by 
Zaccagna and Mattirolo; but the results of the detailed survey and 
the consequent extension of microscopic work, notably by Franchi, 
Novarese, and Stella, have led to revised and more precise definitions, 
more especially in reference to the amphibolic and prasinitic series 
which Gastaldi included indiscriminately in his ‘amphibolic’ or 
‘“magnesian schist zone’. 
On the rational ground that not amphibole, i.e. hornblende, but 
triclinic felspar is the most prevalent constituent of pietre verdi, the 
revised nomenclature divides all the basic rocks of the Piémontese 
Alps into three groups on a felspathic basis, viz. rocks in which 
felspar, as a constituent element, is essential, subordinate, or absent. 
In the following table I have enumerated only the principal, most 
diffused rocks, without their schists, their infinite graduations, and 
their often overlapping varieties and combinations.’ . 
PIETRE VERDI ROCKS. 
I. Rocks with primitive elements. 
(1) Essential. Diorite, diabase, porphyrite, gabbro, and their 
FELSPAR varieties. 
(2) Subordinate. Felspathic lherzolite, felspathic hornblendite. 
(3) Absent. Lherzolite and peridotite, hornblendite. 
1 G. Striiver, ‘‘Cenni sui graniti massicci delle Alpi Piémontesi e sui minerali 
delle valli di Lanzo’’: Mem. descr. Carta geol. d’Italia, 1871, p. 37 ef seq. 
M. Baretti, ‘‘ Studi geol. sul gruppo del Gran Paradiso’’?: Mem. Acc. Lincei 
Torino, vol. i, p. 197 et seq., 1876-7. L. Bucca, “‘ Appunti petrogr. sul gruppo 
del Gran Paradiso’’: Boll. R. Com. geol., 1886, p. 449 eti seq. 
2 The table is founded on the nomenclature worked out by Novarese and 
Franchi, Boll. R. Com. geol., 1895, p. 164 et seq. and p. 181 et seq.; but 
I have arranged it somewhat differently so as to group the rocks with primitive 
and those with secondary elements separately and more prominently. 
