84 PRINCIPLES OF STRATIGRAPHY 



former extensive sand dunes now covered over by vegetation must 

 further be regarded as indicative of a change of climate. 



Stratigrapiiic E\ir)ENCE OF CiiANGK OF Climatf. A careful 

 consideration of the lithic characters of the strata of a given region 

 may furnish evidence of changes of climate in the successive geo- 

 logic epochs. Thus ancient alluvial fans and delta deposits inter- 

 calated between marine sediments are not only evidence of eleva- 

 tion followed by subsidence, but may also indicate a change in cli- 

 matic conditions. This is especially the case when such alluvial 

 fans are characterized by coarse waste, or by red color. The red 

 color of such continental deposits as the Longwood shale of the 

 late Siluric, the Catskill and Old Red sandstones of the Devonic, 

 the Mauch Chunk of the IMississippic, and the Newark and Red beds 

 of the Trias, have been regarded as indicating more or less arid con- 

 ditions during the formation of those deposits. Huntington (51) 

 has described the alternations of red and green strata exposed in 

 the uplifted and dissected bottom of the Pleistocenic lake Seyistan 

 in eastern Persia. The red or pink strata are thick beds of clays, 

 silts, and fine brown sands of a very continuous and uniform char- 

 acter, traceable for miles, even though varying in minor details. 

 They show evidence of exposure to the air, under conditions which 

 prevented extensive development of vegetation. The white or 

 greenish layers, on the other hand, are solid beds of clay, lined above 

 and below by somewhat more sandy films. These green clays were 

 deposited during the periods of expansion of the lake, while the red 

 beds indicate exposure to a dry climate followed by oxidation dur- 

 ing periods of retreat, when arid conditions prevailed. There are 

 here indicated ten epochs of expansion of the lake (the arsial 

 epochs) and ten of contraction (the thesial epochs). The Moen- 

 copie beds of Utah and Arizona (Permic) show a similar alterna- 

 tion of red and greenish beds and indicate a similar pulsation of the 

 climate of the Permic. 



Arid conditions are also shown by the arkose character of many 

 rocks, as, for example, the Torridon sandstone of western Scotland, 

 where the feldspar crysta,ls are scarcely weathered, and the Newark 

 sandstone beneath the Palisades of the Hudson. Arid conditions 

 are further indicated by the occurrence of beds of salt and gypsum, 

 and the same thing is shown by extensive deposits of wind-blown, 

 cross-bedded sands such as are found in the White River beds (Ju- 

 rassic) of Arizona. The extensive development of coal swamps and 

 marshes may indicate a change to cooler and moister climate, while 

 tillites, coarse boulder conglomerates and striated rock and boul- 

 der surfaces indicate glacial conditions. 



