TERTIARY CONTINENTAL HYDROCLASTICS 631 



ments forming the "red Mollasse" are frequent in the upper non- 

 marine part of the Ohgocenic Mollasse, especially in Switzerland, 

 but occur also in upper Swabia. The red beds are generally fol- 

 lowed by conglomerates with pebbles ranging in size from that of 

 an egg to that of a man's head and locally kown as Nagelfluh. 

 These in places reach a great thickness. Away from the Alps the 

 material becomes finer. The Miocenic Alollasse succeeding this 

 often begins with calcareous beds to a large extent formed of the 

 shells of the land snail, Helix rugulosa (Rugulosa limestones). 

 This is followed by a series of loose sands, glauconitic sandstones 

 and conglomerates (Nagelfluh) several hundred meters thick in the 

 southern part of the region. The lower part of this series is again 

 purely marine, but the upper part is once more brackish and non- 

 marine, beds containing Cardium sociale, Melanopsis, etc., marking 

 the brackish portion ; sands with Paludina, Unio, and Chara fruits, 

 marking the lacustrine and fluviatile. These series constitute the 

 Middle Mollasse. The highest beds finally forming the Upper 

 Mollasse of Upper Miocenic age are again wholly non-marine. 

 They consist of sands, clays, marls, occasional thin beds of brown 

 coal, local volcanic tuffs and especially non-marine limestones. 

 These so-called Sylvana limestones consist of the shells of the land 

 snails Helix sylvana and H . inflexa, of those of the pond and river 

 snails Planorbis, Limnaeus, etc. Bones of the mastodon also occur. 

 Other beds contain an abundance of the shells of the river and 

 lake molluscs, Unio, Anodonta, Limnjeus, Melania, Melanopsis, 

 etc. Local deposits of thin marly limestones with plant (maple, 

 poplar, etc.) and insect remains also occur, as in the celebrated de- 

 posit of Oningen on the Lake of Constance (Bodensee) and 

 local deposits like those of the Steinheim basin with its sands 

 filled with Planorbis, Helix, and land vertebrates. Remains of 

 terrestrial vertebrates abound in all of these deposits. 



Since the axis of the Alps is parallel to the direction of the rain- 

 bringing winds, both sides receive an abundant rainfall, though 

 within the mountains are dry valleys. The extensive formation of 

 the Mollasse, partly of subaerial origin on the northern side of the 

 Alps, suggests a different condition during Tertiary times, so as to 

 result in a more arid condition on the north, or, at any rate, in con- 

 ditions which would favor the formation of extensive alluvial fans. 



Mesozoic Examples of Continental Hydroclastics. 



The Potomac Foniiation of the Atlantic coast of North America 

 represents a series of delta and flood plain deposits ccmiparable 



