FOLDS RESULTING FROM VERTICALLY ACTING FORCES 343 



Linear terrace folds. — On long lines of movement in some cases 

 apparent at surface. Not due to concealed faulting. Seldom 

 have topographic expression. 



Monoclines by faulting. — ^Simple monoclinal folds due to con- 

 cealed faults. In some cases, with topographic expression. 



This article has been written with the purpose of directing 

 attention to the importance, economically, of the structures 

 described. In the Michigan basin little encouragement is offered 

 for their exploration, chiefly on account of the long continued 

 exposure of this area to erosional forces, continuing as it apparently 

 did from Pennsylvanian to Pleistocene time, thus keeping open 

 the path for the escape of fluids along the axes of the radial linear 

 folds and preventing the deposition of any impervious layers above 

 them but with a different Tertiary or Mesozoic history areas of a 

 similar nature would offer many interesting possibilities. 



