TROUGH FAULTS AND CLAY SEAMS. 



239 



a more or less distinctly wedge-shaped portion to be displaced. These 

 are commonly called trough faults. One observed in the Appanoose 

 shaft in the|southern part of the state is represented in figure 5. Here 



FiGVUE^o. — Trough Fault in Appanoose Shaft, southern Iowa. 



the strain has evidently been relieved partly through a major slip and 

 partly through a minor one. 



Clay Seams. — Simple fissures or ruptures are of not infrequent occur- 

 rence in the Coal Measures of Iowa. They are merely a separation of 

 the different parts of the vein without apparent displacement. The 



Figure 6.— Clay Fissure in Thistle Mine, Appanoose County, Iowa. 



fissures are usually more or less vertical, with often very irregular bor- 

 ders, though sometimes they may be inclined at considerable angles. 

 In the majority of cases clay fills the fissures which may be from a few 

 inches to a foot or more in width, or sand may occupy all the space 



XXXIII— BuLr-. Geoi, Soc. Am., Vol. 5. 1893. 



