124 /• A. UDDEN 



the ripple marks from a half to one inch deep. The wind stirred 

 the surface of the water gently into small waves, and the ripple 

 marks in the sand were seen to be building, under the influence 

 of these waves. 



Fig. 2 shows some ripple marks in a fine silty sand of the marine 

 Jurassic, near Minnekahta, South Dakota. They measure i^ inches 

 from crest to crest and have an average depth of iifteen-hundredths 



Fig. i. — Ripple marks in Comanchean sandstone, from Pecos County, Texas. 

 Natural size. 



of an inch. These ripple marks are unsymmetrical, their longer 

 slopes bearing the average ratio of 152 to 100, to the shorter slopes. 

 A mechanical analysis of the sand in this rock was found to be, 

 roughly, as follows: 



Diameter of Grains Percentages 



in Millimeters by Weight 



1/2 -1/4 Trace 



1/4 -1/8 55 



1/8 -1/16 30 



1/16-1/32 15 



Some large-sized ripple marks occur in the Ordovician dolomites 

 at Utica, in Illinois. In the old entries where cement rock long ago 



