TWO MEMBERS OF COAL MEASURES. 81 



These two categories are sharp]y contrasted lithologically, 

 slratigraphically and faunally. The first is characterized by 

 the rocks being piedominantly clay-shales and sandstones, 

 with practically no limestones. The individual beds have usu- 

 ally a ^ ery limited extent, and replace one another in rapid 

 succession, both laterally and vertically. The sandstones often 

 form great lenticular masses, sometimes deeply channeled 

 on the upper surface, the excavations being tilled with Coal 

 Measure clays. These and many other phenomena attest a 

 constantly shifting phore line and shallow waters. The fossils 

 contained are nearly all brackish water forms or shore species. 

 Eemains of pelagic organisms are not numerous. 



On the other hand, the second class of deposits is made 

 up largely of calcareous shales, with heavy beds of limestone. 

 The layers are evenly bedded, and extend over very consider- 

 able distances. The faunas are chiefly composed of strictly 

 open-sea forms. 



As the conditions of deposition were evidently those of 

 a slowly sinking shore, the marginal deposits as a whole prac- 

 tically underlie the open-sea formations, the former being re- 

 garded as the Lower Coal Measures and the latter as the Upper 

 Coal Measures. At the same time it must be remembered that 

 this does not necessarily imply that the " lower " measures are 

 to be considered much older than the " upper," but rather that 

 along the great and successive planes of sedimentation differ- 

 ent beds of the upper and lower divisions were laid down 

 contemporaneously. 



While the general divisions of the Coal Measures may be 

 readily recognized, it does not seem advisable to draw over 

 the whole region an exact line of demarkation between the two 

 formations, until the evidence of the faunal studies already 

 begun has been fully taken into consideration and a comparison 

 of the results of the different methods of solving the problem 

 is made. 



With this idea of the Coal Measures of the interior basin, 

 the limits of the two formations in Missouri, Iowa and the dis- 



