712 PRINCIPLES OF STRATIGRAPHY 



into the clay ball which then assumes the appearance of a worn 

 conglomerate fragment. As such it may be embedded in the suc- 

 ceeding formation. Frass (lo) records such boulders with shell 

 fragments for pebbles, from the Jurassic of Spitzbergen. 



9. Rain Prints. Partly dried clay surfaces and to some ex- 

 tent those of other lutaceous deposits, when exposed to a short, 

 sharp rain, will receive the impressions made by the striking rain 

 drops. When the rain drops strike obliquely a low rim around part 

 of the impression shows where the mud was displaced more 

 strongly, and therefore the side away from that from which the 

 rain slanted. Thus the more pronounced marking is always on the 

 obtuse side of the intersection of the surface and the path of the 

 rain drop. A replica in relief of this impression is found on the 

 under side of the layer of mud spread next above. This feature is 

 eminently characteristic of continental mud deposits, its preserva- 

 tion on the sea coast being a matter of doubtful probability. Im- 

 pressions apparently of this character have, however, been re- 



FiG. 140. Plan of Eolian rip- Fig. 141. Diagram showing direction of 

 pie-marks in fine sand. currents and vortices in the formation 



(After Walther.) of ripple-marks. (After Darwin.) 



ported from the marine (shallow water) limestones of the Cin- 

 cinnati region. ( Perry-i 8 : Ji-p, Fig. i.) 



10. Ripple Marks. These are rhythmic undulations or waves 

 of the sand due either to the motion of the air or of the water. 

 Two types of ripples are recognized: i. Current ripples, formed 

 by the wind on the surface of sand dunes, etc., or by currents of 

 water in shallow basins ; and 2. Oscillation ripples, formed by the 

 vortices of water in the sands at the bottoms of shallow stationary 

 water bodies. 



"A current of water flowing over a bed of sand reacts on any 

 prominence of the bed. An eddy or vortex is created in the lee of 

 the prominence, and the return current of this vortex checks travel- 

 ing particles, causing a growth of the prominence on its down- 

 stream side. At the same time the upstream side is eroded, and the 

 prominence thus travels downstream. It is a subaqueous dune. Its 

 upstream slope is long and gentle ; its downstream slope is short and 

 steep." (Gilbert-i2:7?7.) The natural mold of this type of ripple 



