258 GENERAL GEOLOGY. 



is real. A plane so large as this being determined by actnal 

 observation, lines produced from it in all directions will of 

 themselves give a good general idea of the position of strata 

 of the Upper coal-measures of southwestern Iowa, until they 

 reach points at which we know other dips take place, even if 

 the opportunity of actual observation of the strata at the 

 surface cannot be obtained. Fortunately, such opportunities 

 are not wanting and their results confirm these deductions. 



5. PRACTICAL CONCLUSIONS. 



Usually it is of little practical importance what becomes of 

 a formation after it has passed beneath the surface, because 

 the labor of obtaining at any considerable depth in the earth 

 the more common materials it may furnish is too great to 

 render such labors profitable. Coal, however, is so valuable, 

 indeed, so indispensible, that men are not content with 

 seeking it among the coal-bearing strata where they occupy 

 the surface alone, but they also pursue it into the earth as far 

 as it is practicable to go, seeking it far beneath surfaces upon 

 which it does not appear, and where its existence would never 

 have been suspected, except for the knowledge geology gives 

 of the extensions of formations beneath each other. 



By reference to the geological map-model in the first part 

 of this volume, it will be seen that the sheet which represents 

 the coal-bearing formations passes beneath others. That 

 sheet upon which is printed the portion representing the 

 southwestern part of the State, represents also the Upper 

 or unproductive* coal-measures which rest upon and cover 

 up a large part of the two lower, or productive coal forma- 

 tions. When the latter come to the surface, as they do in all 

 that part of the State represented by the uncovered part of 

 the sheet which itself represents the formations in question, 

 the search for coal may be prosecuted with reasonable hope 



*This formation really contains one thin bed of coal as previously explained, but it 

 is so unimportant that the formation is often referred to as "barren, or unproductive," 

 which it really is, as compared with the great productiveness of the Lower coal- 



