1848.] MURCHISON ON THE STRUCTURE OF THE ALPS. 2/9 



of the Arno, afford sections of macigno, and, whether of grey or 

 greenish-grey colours, it is usually the same slightly micaceous and 

 feebly calcareous sandstone with grains of black schist, the fine build- 

 ing-stone of this region. At Ripa-fratta and other places to the north 

 of the Pisan hills, this macigno is seen to dip away from all the chief 

 underlying calcareous masses. But unfortunately the absence of fossils 

 between the ammonitic group of the latter and the lowest beds of true 

 macigno (the interval being occupied by compact limestone with flints), 

 defeats any attempt at close comparisons *. 



In ascending the valley of the Arno above Florence, and particu- 

 larly between Ponte Sieve and Incisa, strong bands of alberese lime- 

 stone undulate in rapid flexures or anticlinals, and dip under vast 

 thicknesses of macigno, which roll over rapidly to the W.N.W. and 

 E.S.E. At Monte Consuma these macigno rocks contain two or 

 three courses of nummulitic limestone, as Professor Pilla has already 

 indicated. 



The grand masses of macigno which occupy the sides of the upper 

 valley of the Tiber near Arezzo and thence range down to the envi- 

 rons of Perugia may be followed to the flanks of the highest Apen- 

 nines, where they are seen to repose on the secondary limestones. 

 Between Arezzo and Perugia the macigno is copiously developed, 

 forming the hills on the eastern shores of the Thrasymene lake ; it 

 there clearly alternates with subordinate calcareous bands, and is itself 

 often slightly calciferous. It is here near the centre of a vast trough, 

 the limits of which are the secondary limestones of Monte Cetona 

 on the west and the Apennines on the east. I was not able to 

 satisfy myself, by any absolute superposition to strata with creta- 

 ceous fossils, whether these rocks are really of lower tertiary age ; 

 but my impression is that they are simply prolongations of the 

 eocene macigno of Arezzo and Monte Consuma, in which nummu- 

 litic bands occur. In examining them I was reminded of an obser- 

 vation made by my lamented friend M. Alex. Brongniart, who, 

 when I first showed him characteristic specimens of the upper 

 Silurian rock of Ludlow, exclaimed that they were true "macigno." 

 I assert that the small micaceous, slightly calcareous, earthy sand- 

 stones, breaking to a bluish heart within, and weathering to a dirty 



* *' Sulla costituzione geologica del monti Pisani, memoria del Prof. Cav. Paolo 

 Savi, Pisa, 1846." Placing the nummulitic limestone as the uppermost bed of 

 the cretaceous rocks, Professor Savi shows that it reposes on alberese with fucoids 

 with and without flints, macigno sandstone, argillaceous schists, and mottled 

 limestone with fucoids. Beneath this upper group are other and darker-coloured 

 limestones, with flints and fucoids, which form the base of the cretaceous rocks. 

 He then classes in the upper member of the Jurassic series a light grey limestone 

 (also with flints), which, he says, passes into and contains some of the fossils of 

 the red ammonitic rock, about which no doubt exists. His lower Jurassic or lias 

 is made up of whitish limestone with fossil bivalves and turriculated shells, of a dark 

 grey limestone, also with some obscure fossils, and lastly, at its base, of the 

 *' Verrucano." The only horizon clearly marked by its secondary fossils in all this 

 series is the " ammonitico rosso ;" but judging from the overlying position of the 

 nummulites in the south of Italy, as well as in the north, it is probable that the great 

 mass or lower portion of alberese limestones is, I repeat, really cretaceous. See a 

 translation of Prof. Savi'sMemoir in Quart. Journ. Geol. Soc. vol.iii. part ii. p. 1-10. 



