592 PRINCIPLES OF STRATIGRAPHY 



this zone is exactly like the sand forming the banks of the great 

 rivers, but in a more or less consolidated condition." (Medlicott 

 and Blanford-38 :5i'^.) The fossils of this formation are of fresh 

 water or of terrestrial types exclusively, and they, as well as the 

 nature of the deposit, point the continental origin of this forma- 

 tion. "The mountain torrents are now in many cases engaged in 

 laying down great banks of shingle at the margin of the plains, just 

 like the Siwalik conglomerates ; and the thick sandstones and sandy 

 clays of the Tertiary series are of just the same type of form and 

 composition as the actual deposits of the great rivers." (Medlicott 

 and Blanford.) 



In the Salt Range of the Punjab these alternating gray and 

 greenish sandstones and the red and light brownish orange clays 

 "are from seventy to a hundred and twenty feet in thickness, be- 

 ing very frequently about a hundred feet each, but some zones 

 are much thicker." (Wynne-62 :7o6'.) 



Depth of Compound Continental Deposits. The great amount 

 of material which may accumulate in deserts as the product of 

 combined creep, torrential and eolian deposit is seen from a well 

 boring near Ashkabad, Turkestan (Walther-57 r/oj), which pene- 

 trated sands, clays and gravels to a depth of 666 meters without 

 finding rock bottom. No organic remains were found, and the 

 character of the material is similar to that of the surface deposits 

 of the Transcaspian desert, from which it is inferred that this entire 

 mass of over 2,000 feet of sediment accumulated under climatic 

 and topographic conditions similar to those now prevailing. 



Chatter or Percussion Marks. A characteristic feature of many 

 boulder or cobblestone deposits of modern time as well as of 

 former periods is the presence of numerous crescentic chatter or 

 percussion marks on the finer grained and well-rounded pebbles, 

 especially porphyries, quartzites, and the like. These have much 

 the form and size of impressions made by the end of a finger nail 

 in soft clay, and are due to the violent impact of one rounded 

 pebble or boiilder upon another. Such marks are plentifully pro- 

 duced by the impact, one upon another of the hard, fine-grained 

 pebbles used for grinding in the revolving cylinders of cement mills 

 and other works. 



Organic Remains in Torrential Deposits. On the whole, or- 

 ganic remains will be few or absent in coarse torrential deposits. 

 Even tough masses of wood will be shattered and completely anni- 

 hilated. As a result, such deposits will be free from organic re- 

 mains, though, of course, with increasing fineness of the sediment 

 the possibility of the preservation of such remains increases. Since 



