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. 



