84 GEOLOGICAL FORMATIONS. 



made up of the coarse sand are much thicker than the other 

 layers of similar composition, but they become attenuated in 

 all directions from the center, much more rapidly than the finer 

 and clay-bearing portions. 



In the Lower Goal Measures, the limestones pUy a very 

 unimportant part. They consist merely of a few thin bands, 

 which, however, are the most persistent and easily recogniza- 

 ble over wide areas of any part of the separate horizons. They 

 are usually fragmentary or nodular, very impure from a large 

 admixture of clayey material, and often more or less highly 

 fossiliferous. In the Upper Coal Measures the limerocks 

 become important members. ( Plate x.) Being largely open- 

 sea deposits, they have a wide geographical extent. The strata 

 are more compact, more heavily bedded and much thicker 

 than in the lower division. Abundant fossils are enclosed in 

 these rocks, especially where they begin to pass into marly 

 clays. Here they are usually in an excellent state of preser- 

 vation. 



Economically, the coals form by far the most important 

 deposits of the Carboniferous. Stratigraphically, they are of 

 small import. The seams vary from a few inches to several 

 feet in thickness. They are not disposed in a few continuous 

 layers over the entire area, but in numerous lenticular masses, 

 from a few hundred yards to several miles in diameter. A sin- 

 gle [horizon may thus contain several or many of these lens- 

 shaped beds of greater or less extent. Eecognizing this fact, 

 the aggregated amount of coal is probably far in excess of 

 what has been hitherto commonly supposed. 



The general stratigraphical features of the Missouri Coal 

 Measures have been so clearly set forth lately,* that some of 

 the leading suggestions may be summarized in the present 

 connection. 



The Coal Measure rocks of Missouri are arranged in a ser- 

 ies of strata which have generally a slight, undulating westerly 

 dip, such as the uppermost rocks are at the surface in the 



* Winslow: Geol. Sur. Missouri, Prelim. Rep. Goal; pp. 22-32. 



