File:The Quarterly journal of the Geological Society of London (12645656714).jpg

1848..
MURCHISON ON THE STRUCTURE OF THE ALPS.
281
wards into the upper beds of the great cretaceous limestones, of which
all the surrounding mountains are composed, and in the grottoes of
which St. Benedict established his famous monastery. Here, as at
Olevano, the beds between the solid hippuritic limestones and the
macigno are sandy or siliceous, dirty or yellowish white limestones.
Fig. 35.
Sabine Hills.
Macigno. g
Pliocene or Post-pliocene
•p t g. Macigno.
liocene. .^ j,_ Nummulite limestone.
Cretaceous, d. Hippurite limestone.
Hippunte limestone.
z. Conglomerate.
with nummulites and pectens. Near Agosta, lower down the valley
of the Teverone, there are extensive quarries of this macigno where the
rock, being deeply cut into, is blue-hearted, of concretionary forms
on the great scale, and quite undistinguishable from the Pietra
forte" of Florence. The strata dip slightly across the valley and
appear to plunge under the massive limestone cliffs of Agosta, but
this appearance is fallacious, and is simply the result of one of the
numerous faults of the chain ; for the macigno is inclined at 1 0° or 1 2°
only, and the secondary limestone plunges 4.5°. It is, I apprehend,
from such examples that the supposed intercalation of the overlying
macigno with the secondary limestones has been supposed to exist.
Judging from the section and brief description by MM. Alessan-
dro Spada and Orsini* of the rocks between the watershed of the
Apennines and the Adriatic in the parallel of Ascoli and the Tronto
river, it would appear that there is there a much greater simplicity
of structure than on the western side of the axis. This symmetrical
disposition may be accounted for by the absence of those eruptive and
volcanic rocks which are so abundant along the western slopes of the
chain. Although I was prevented from visiting the Adriatic shores
by the political state of the country, I cannot refer to the sections of
Spada and Orsini without suggesting that one essential phsenomenon
of that region is in accordance with my own views. Their diagram
shows a concordant passage from the limestone called "majolica"
into overlying limestone with nummulites, and thence upwards
through grey impure limestones with fucoids into macigno. Now
whether this majolica be, as I suspect, neocomian and not Jurassic
(as they believe), and whether there be or be not any representative
of the white chalk, we have clearly an ascending succession, in which
the macigno is the highest mass, and is overlaid only by tertiary mio^
cene strata with gypsum. The question after all is, what are the
fossils which are there associated with true nummulites 1 and from

• Bull. Soc. Geol. Fr. 2 Ser. t. ii. p. 408, 1845.