TORRENTIAL DEPOSITS 591 



of river deposits is practically unlimited, and depends on the source 

 of supply of material and the depth of the valley in which it is 

 deposited. Where downwarping of a piedmont belt occurs, so as 

 to form a geosyncline, the thickness of the formation may become 

 enormous. Torrential deposits in Calabria, Italy, have been found 

 1,500 feet or more in thickness and including boulders of many 

 petrographic types, often exceeding a foot in diameter. The Al- 

 hambra formation of Spain is at least 1,000 feet thick and consists 

 mainly of river deposited pebbles distinctly water worn and varying 

 in size from a fraction of an inch to six inches or more in length. 

 Some of the partly dissected, but still unconsolidated torrential 

 deposits fringing the Front Range of the Rocky Mountain region 

 contain boulders many feet in diameter. Some torrential deposits 

 may be entirely composed of rounded boulders with only enough 

 sand and fine gravel to fill the interstices. Portions of the Old Red 

 sandstone of Scotland furnish splendid examples of fossil boulder 

 beds of this kind. 



The Siwalik formation of India furnishes an excellent example 

 of a sub-recent fluviatile deposit of great thickness. It skirts the 

 southern border of the Himalayas, forming the Siwalik Hills. The 

 strata have been uplifted by the latest movement in this region and 

 exposed by erosion. The thickness of the formation is upward of 

 15,000 feet, and, except at the base, where it is characterized by 

 passage beds from the underlying marine Sirmur group, it is of 

 non-marine origin throughout. In age it is said to range from 

 Miocenic to Quaternary, though most of it belongs to the Pliocenic. 

 In this formation "sandstone immensely predominates . . 

 and is of a very persistent type from end to end of the region and 

 from top to bottom of the series. Its commonest form is indis- 

 tinguishable from the rock of corresponding age known as Molasse 

 in the Alps, and is of a clear pepper and salt gray, sharp and fine in 

 grain, generally soft, and in very massive beds. The whole Middle 

 and Lower Siwaliks are formed of this rock, with occasional thick 

 beds of red clay and very rare thin, discontinuous bands and 

 nodules of earthy limestone, the sandstone itself being sometimes 

 calcareous and thus cemented into hard nodular masses. . . . 

 In the Upper Siwaliks conglomerates prevail largely; they are often 

 made up of coarsest shingle, precisely like that in the beds of the 

 great Himalayan torrents. P)rown clays occur often with the con- 

 glomerate, and sometimes almost entirely replace it. This clay, 

 even when tilted to the vertical, is indistinguishable in hand speci- 

 mens from that of the recent plains deposit ; and no doubt it was 

 formed in a similar manner, as alluvium. The sandstone, too, of 



