191-'.] STEVENSON— THE FORMATION OF COAL BEDS. 541 



covered in many places with willows and thick underbrush, so that 

 the current's speed is reduced and there is little erosion of the 

 dikes. The deposits on the plain of the upper Po are irregular and 

 cross-bedded, consisting of cobbles, coarse gravel and pebbles with 

 occasional wedges of sand. At times, local deposits of stratified 

 silt and clay are seen, such as that near Turin, covering 40 acres. 

 The deposits are more regular along the lower Po, much of the 

 material being clay or fine sand, often laminated. In extended 

 exposures, long, flat lenses of sand are shown, which are enwrapped 

 by the finer sediments. 



The plain of northern Italy receives drainage and deposits from 

 the Alps and Appenines by way of the Po and its tributaries; from 

 the Tyrolese Alps by way of the Adige ; while smaller streams, 

 flowing directly to the Adriatic, contribute their share toward ex- 

 tension of the plain. Taramelli"° has gathered the information 

 bearing on the development of the plain, with its sands, clays and 

 occasional coral limestones. At the beginning of the Pliocene, it 

 was in great part dry land, for great valleys were excavated, in 

 which gravels were deposited. The topography, in its broader fea- 

 tures, was well-defined at that time. During the later Pliocene, the 

 region was invaded by the sea and deposits, termed marine Pliocene, 

 were laid down. These have been recognized in much of Italy, 

 which must have been an archipelago. But at the close of the Plio- 

 cene, irregular differential elevation took place, as appears from the 

 altitudes correlated by Taramelli. It is clear that, after the Miocene, 

 a great area was converted into dry land, to be brought again under 

 the sea, but afterwards to be elevated in some localities to 700 

 meters above that level, while in others it is still below it. These 

 Pliocene beds are the terraces of diluvial deposits. 



De Collegno^*^ has shown the wide extent of pebbly and sandy 

 deposits in northern Italy and the relations of those deposits to the 

 rivers. On the plain of Milan, the pebbles are often consolidated 

 into a conglomerate, which is exposed along the river and in rail- 



140 'p 'Taramelli, " L'Epoca glaciale in Italia," Atti Soc. Ital. Progr. Sci., 

 Riunion IV., 1910, separate, pp. 5, 6. 



"' De Collegno, " Note sur le terrain erratique du revers meridional des 

 Alpes," Bull. Soc. Geol. de France, II., Vol. II., 1845, pp. 284-286. 



