EROSION OF DISTURBED STRATA 843 



the erosion of the dome ( I'ig. 215, A. h d e) would progressively 

 come to rest upon the beveled edges of older and older strata, in the 

 direction of the center of the dome, the relation being an uncon- 

 formable one. This same type of structure might, of course, be 

 produced by a gradually retreating sea from a rising island, so that 

 each succeeding stratum reaches to a less distance than the pre- 

 ceding. In that case, however, the ends of tlie successive strata 

 would show shore characteristics, and fragments of the lower might 

 be enclosed in the higher formations. 



The Cincinnati and Nashville domes are typical examples of low 

 domes with very gently inclined strata formed and eroded during 

 Palaeozoic time. As pointed out repeatedly by Dr. Foerste (15) and 

 others, the lower Siluric (Niagaran) strata found on the flanks of 

 the dome conform in character to the first of the two cases cited, 

 all the evidence pointing to the fact that the Niagaran strata for- 

 merly extended across the domes, which therefore formed in late 

 Siluric or early Devonic time. As the eroded edges of the strata 

 are disconformably overlain by Mid-Devonic limestones, or by 

 Upper Devonic or younger black shales, it is evident that the ero- 

 sion of the dome preceded Mid-Devonic time. This probably oc- 

 curred during the Helderberg period, while the greater part of 

 North America was above sea-level. 



Subsequently to the deposition of the higher Paljeozoic over this 

 pre-Devonic truncated dome, one or more additional domings took 

 place, followed by erosion which again exposed the lowest central 

 strata. 



4. Erosion Features on the Anticline. 



The anticline dififers from the dome chiefly in the fact that the 

 longitudinal axis is many times longer than the transverse. Since 

 the anticline must come to an end in either direction by a downward 

 pitching of the axis, the characteristics of the simple anticline may 

 be considered those of an excessively elongated dome. While domes. 

 however, generally occur singly, anticlines occur most commonly in 

 series, a number of parallel anticlines being separated by synclines. 

 The erosion structure of such anticlines is in general similar in 

 each anticline to that of the dome, except that the subsequent val- 

 leys and the hog-back ridges are parallel, instead of circumferential, 

 and the transverse consequent gorges in the ridges are parallel in- 

 stead of radial. The most important difference' lies in the duplica- 

 tion of the structure in each anticline and its complication by the 



