Dr. Dw Riche Preller—Crystalline Rocks, N. Prémont. 358 
formation. It was first pointed out by Gastaldi as early as 1871,' 
and cutting, as it does, right across the crystalline area, is still the 
most extensive and instructive, as it is also the most easily accessible 
section of the whole region. 
IV. Conctiustrons. 
The description given in this and the preceding papers of the six 
principal crystalline rock-areas of the Piémontese Alps, condensed 
and summarized though it necessarily is, will, I think, convey an 
‘ adequate idea of the eminently characteristic geological and petro- 
logical features of each group. Though unique in their collective 
variety they reveal a certain uniformity of phenomena which from my 
own observations leads to the following conclusions :-— 
1. The pietre verdi, primitive and secondary, appear at all levels 
from the valley floors of 400 to the highest points 4,000 metres in 
altitude. Except the prevalence of pietre verdi with primitive 
elements in the minute gneiss and mica-schist, and of those with 
secondary elements in the calc-schist formation, there is no order of 
succession, superposition, or distribution. 
2. The basic pietre verdi of all the areas, as well as the acid rocks 
of the Ivrea belt, are of eruptive origin, but not generally intrusive 
in the cale-schist and mica-schist formations. They appear in the 
latter not as irruptive, or as angular injections with apophyses, but 
as intercalated, lenticular, alternating, and concordantly stratiform 
planes and masses which sometimes produce the effect of pseudo- 
intrusive phenomena. There are frequent passages between the 
eruptive rocks, and from them to the sedimentary rocks and vice 
versa; but there is no reliable or conclusive evidence of contact 
‘metamorphism. 
3. The close association of rocks of different origin is in the first 
instance the result of repeated compenetration of eruptive viscid 
masses and of sedimentary deposits in course of consolidation. After 
‘the resultant formation of conformable, alternating, interstratified 
planes as a submarine process of long duration, the rocks thus 
associated passed through the second phase of being raised and folded 
simultaneously during repeated earth-movements. The eruptive 
rocks of the Piémontese Alps are therefore of the same age as the 
infolding cale-schist and mica-schist formations respectively. 
4. The process of pressure-heat- and hydro-metamorphism of both 
the sedimentary and eruptive rocks, already in progress before their 
emergence, continued not only during the repeated periods of raising 
and folding, but during the repeated Pliocene and Pleistocene 
glaciations under the long-continued, superincumbent, and shearing 
pressure of the ice, and the hydrating, decomposing action of perco- 
lating water, together with infiltrations from the Pliocene sea of the 
Po Valley. ‘The dissolving action of the water thus imprisoned and 
circulating was intensified by the high temperature which it took 
from the rocks, in conjunction with thermal, highly magnesian 
springs at various depths. 
1 Gastaldi, ‘‘ Studi geol. Alpi Occid.’’: Boll. R. Com. geol., 1871, p. 3 
et seq., p. 17. 
DECADE VI.—VOL. III.—NO. VIII. 23 
