2/0 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 13, 



I will only add, that I believe the deep bay of La Spezia has been 

 excavated in a synclinal trough of macigno, whilst the hard inferior 

 limestones and marbles *have resisted, and form broken anticlinals on 

 either side of the bay. On the east we see, in fact, another and broader 

 parallel synclinal trough, the valley of the Magra, which has been 

 excavated in similar macigno, covered by some remnants of overlying 

 tertiary. Again, when we traverse the great anticlinal of the Apuan 

 Alps, and descend into the valley of the Serchio, we meet with a third 

 and similar trough, on the eastern side of which the lower portion of 

 the macigno rises up into mountainous elevations, of which hereafter. 



I have commenced with this superficial sketch of the general ar- 

 rangement of the rocks in this northern tract of Italy, not merely 

 because the oldest known limestones in the chain of the Apennines 

 are brought out in parallel anticlinals, but because the outline of 

 ridge and furrow, here so clearly developed, is a key to the general 

 structure of the Apennines. In truth, the Italian peninsula is not 

 characterized by one central backbone or central axis, but is made up 

 of a frequent repetition of.axes, the rocks composing which are some- 

 times much altered, often dismembered, and frequently covered over 

 by younger sediments ; the older portions of the series being only seen 

 at intervals as we follow the chain from N.N.W. to S.S.E. Thus, 

 Jurassic rocks have not been detected in the broad and mountainous 

 undulations which extend over the principalities of Lucca and Parma, 

 or the country of Genoa, a region almost entirely occupied by lime- 

 stones and macigno sandstone, whose age I shall presently endeavour 

 to explain. 



In the northern portion of the Tuscan Maremma, I examined, in 

 company with M. Pilla and M. Coquand, the axis of Jurassic rocks 

 which, at Campiglia, are converted into domes of crystalline white 

 marble in the contiguity of points of granite. This marble throws off 

 on its flanks a compact thin-bedded limestone with encrinites over- 

 laid by schists, with Posidonia Jossice, the latter being abruptly trun- 

 cated against masses of macigno and alberese. This Jurassic group 

 is evidently the prolongation of one of the zones of the above-described 

 Apuan Alps and Pisan hills * . The existence of true Jurassic rocks has 

 further been clearly indicated by M. E. de Vecchi in Monte Cetona, 

 on another parallel between the Maremma and the Apennines. The 

 nucleus of this hill is evidently the same Oxfordian limestone with Atn- 

 monites Tat?'icus which constitutes the upper group at La Spezia. 

 Again, in another parallel, the group of mountains between the Ma- 

 remma and Sienna, composed of white, yellow and red marbles (the 

 Montagnuolo Senese), the whole reposing on a conglomerate, probably 

 represent the Jurassic series, since they are overlaid by scaglia ; but 

 no fossils have yet been detected in them. 



In following the chief ridges of the Apennines to the south, but- 

 tresses of true Jurassic rocks are indeed here and there visible, rising 

 out from beneath overlying formations. Ammonites, of the group 



* Some of the phaenomena on the flanks of the granite of Campiglia and the 

 promontory of Piombino are analogous to those of the adjacent isle of Elba, and 

 the rocks are loaded with various crystals of iron ore. 



