Work in the Dakota Group 29 



found lying flat, and others again at various angles. 

 The sand, accumulating through the years, finally 

 became consolidated, and, being in course of time 

 exposed to the air, began to " weather." In the 

 meantime the iron coloring matter of the vegetation 

 had been dissolved out by the water and distributed 

 through the rock mass. As the rock weathers away, 

 the leaf impressions are hardened by the iron that 

 has been dissolved out of the sandy mass by water 

 holding acids in solution. , 



As the soft rock about them continues to wear 

 away, the nodules begin to appear above the sur- 

 face, at first only as bumps slightly elevated above 

 the surrounding rock, but in time as complete con- 

 cretions, with the form of the leaves imprisoned 

 within, which are left standing on pedestals no 

 thicker than a lead pencil. 



Then the first storm of rain or hail breaks them 

 from their moorings ; they become independent, are 

 reduced in size, and constantly hardened, so that 

 often a nodule is almost pure iron ore a fraction of 

 an inch in thickness. 



So the process goes on and will continue until all 

 the leaves within the parent rock have been protected 

 by an iron envelope; and it is this natural process 

 alone which can save these beautiful impressions 

 from falling to pieces when the sand is freed from 

 the rock by disintegration. 



