CAEBONTFEEOUS SYSTEM. 229 



palaeozoic time — now run in valleys that received their initial 

 holloAving at that early period. In addition to this, the 

 receding shore-line of the sea at the close of the St. Louis 

 limestone epoch, and the encroaching one of that in which 

 the Lower coal-measures were deposited, produced other 

 irregularities upon the surface as well as modifications of 

 those previously existing. Thus, when the last named sea 

 had occupied that uneven surface, it of course formed an 

 uneven bed for the sea until the depressions became filled 

 with sediment deposited from its waters ; after which the 

 strata thus formed were more nearly horizontal and con- 

 tinuous. 



The present existence of outliers of coal-measure strata at 

 a distance from the now unbroken border of the coal-field, is 

 largely due to the former existence of these depressions ; for 

 there is presumptive evidence that the coal-field originally 

 extended unbrokenly as far as Scott, Cedar, Linn, Benton, 

 and Humboldt counties, but its deposits there were doubtless 

 originally very thin, too thin, it is believed, to have afforded 

 profitable beds of coal if they had remained, and they were 

 subsequently nearly all swept away by glacial or other 

 agency, leaving only those portions which occupied the 

 hollows as " basin-outliers." The real character of these 

 outliers, as found in Scott, Cedar, Johnson, Benton, Tama, 

 Marshall, Humboldt, Poweshiek, Washington, Louisa, and 

 other counties being popularly misunderstood, has given 

 occasion to much useless expenditure of labor and money in 

 the search for coal. 



As a rule these outliers may be regarded as practically 

 worthless, although they are so unmistakably of coal- 

 measure age. The large outlier extending from Muscatine 

 nearly to Davenport is an exception, and probably others 

 may prove to be so, but even this contains only a single 

 bed of little comparative, although it might be of great 

 positive value under some circumstances. The mining of it 

 will be unprofitable, however, when brought into competition 

 with the main coal-field by railway transportation. The 



