156 Dr. C. 8. Du Riche Preller—Prietre Verdi. 
between the more or less resistant layers of sediment, both in the 
Delta itself and in the sea-bottom just off-shore.! The surface of 
the deltaic deposits is also subsiding, the subsidence being most rapid 
where the Delta is growing most rapidly; and the material is 
presumably becoming more compact and losing its very watery 
condition. These, again, are significant facts, and bear directly upon 
the subject of this communication. We have at once a clue to much 
that was pure surmise—the unstable relations of these sandy, silty, 
and clayey deposits to one another, and the comparative rigidity of 
some of the more sandy beds, which would allow of thrusting and 
buckling within them, although they were still in a pasty condition. 
Il].—Taue “ Prerre Verpr’’ or tHE PifmMontrEsE ALPS. 
By C. S. Du RIcHE PRELLER, M.A., Ph.D., M.I.E.E., F.G.S8., F.R.S.H. 
N a previous paper on the Permian Formation in the Alps of 
Piémont, Dauphiné, and Savoy,? I referred incidentally to the 
large masses of pietre verdi or greenstones which constitute perhaps 
the most striking geological feature of the extensive areas covered by 
the crystalline rocks of the Piémontese Alps in a crescent-shaped 
curve about 200 miles in length from the Maritime range to Monte 
Viso, Grand Paradiso, and Monte Rosa. In the present paper I 
propose to deal more fully, although necessarily within narrow limits, 
with these pietre verdi which, owing alike to their extraordinary 
development, variety, and complexity, to their intimate association 
with each other and with the crystalline sedimentary rocks, and to 
their intricate composition and origin, have for the last fifty years 
presented most interesting problems and passed through many 
remarkable phases of interpretation. As a necessary preliminary to 
a description of the different areas, it will be convenient to briefly 
consider the most recent classification of the crystalline formations of 
the Piémontese Alps generally, and of the pietre verdi rocks in 
particular. 
I. CLAssIFICATION OF THE CRYSTALLINE FoRMATIONS. 
In a short paragraph of the previous paper already quoted, 
I outlined the sequence of the crystalline rocks of the Piémontese 
Alps as evolved by Zaccagna in his revealing memoirs of 1887 and 
1892.8 In this classification he retained Gastaldi’s two principal 
pre-Paleozoic zones or horizons,‘ but with this essential difference, 
that for Gastaldi’s upper or so-called pietre verdi zone he substituted 
1 The mud in the ‘lumps’ is supposed to be squeezed out from between the 
sandy beds. 
2 GEOL. MAG., January, 1916, p. 7; ibid., p. 15. 
3D. Zaccagna, ‘Studi geol. sulle Alpi Occid.’’: Boll. R. Com. geol. d’It., 
1887, p. 346 et seq. ‘‘Riassunto di Osserv. sul Versante Occid. Alpi Graje’’: 
ibid., 1892, p. 175 et seq. 
4 B. Gastaldi, ‘‘ Studi geol. sulle Alpi Occid.’’?: Mem. R..Com. geol. d’It., 
1871, vol. i, p. 3 et seq. ‘‘ Spaccato geol. lungo le valli sup. Po e Varaita’’ : 
Boll. R. Com. geol., 1876, p. 104 et seq. 
