542 STEVENSON— THE FORMATION OF COAL BEDS. [^'ov. i. 



road excavations. He found that, south from Milan, the gravel is 

 too fine for use in maintaining the roads, whereas, north from the 

 city, ample material for that purpose is found just below the surface. 

 From Turin to Verceil, the bowlders rarely exceed 50 centimeters ; 

 but, in ascending from Chivasso toward the valley of Aoste, by Ivree, 

 one finds great blocks of 40 to 50 cubic meters. This deposit is 

 400 meters thick at Ivree, where one often sees 100 meters of con- 

 glomerate ; it is 60 to 80 meters thick on the Adda and some borings 

 on the left bank of the Po have been pushed 60 meters without pass- 

 ing through the detrital mass. 



Martins and Gastaldi"- remark that the alpine diluvium under- 

 lying moraine material is composed of pebbles, which decrease in 

 size as one leaves the Alps. At the foot of the mountains they have 

 a diameter of 40 to 50 centimeters, but at Turin they are rarely as 

 large as a man's head. Small and large pebbles are present together 

 and are mingled with sand and gravel. 



Tacconi^*^ studied collections from about 100 borings in and 

 around Pavia, which is at the junction of the Po and the Ticino. He 

 distinguished readily between the contributions made by those rivers, 

 for glaucophane characterizes the Po material and staurolite abounds 

 in that from the Ticino. Pebbles and gravels are wanting in the 

 exposed deposits, diluvial in the terraces but alluvial in the valley ; 

 in some collections, however, especially in those from the deeper 

 borings, coarse sand and small pebbles are abundant. The usual 

 color of the sands is ashen-gray, but some specimens are yellowish, 

 the tint being due to alteration of ferruginous constituents. These 

 are from different depths and Tacconi conceives that these layers 

 of alluvium may have been exposed to the air for considerable 

 periods before burial under later deposits. The distribution of 

 minerals leads him to suppose that, during the diluvial epoch, the 

 great rivers, descending from the Alps, united in the Lombardy plain 



"" C. Martins et B. Gastaldi. " Essai sur les terrains superficiels de la 

 vallee du Po, aux environs de Turin, compares a eux de la plaine Suisse," 

 Bull. Soc. Geol. de France, II., Vol. VII., 1850, pp. 587-589. 



"' E. Tacconi, " Sulla composizione mineralogica delle alluvioni consti- 

 tuenti il sottosuolo di Pavia e dintorni," Rendic. R. 1st. Lomb., II., Vol. 

 XXXIV, 1901, pp. 873-881. 



