AGE OF AXES SH0W2? 331 



extent from the Precambrian up to the Devonian. In the northern 

 Ozarks there were Devonian axes in considerably different position from 

 those of Carboniferous age. In northern Illinois there is considera])le 

 discordance between axes of the two ages; the principal north-south 

 Devonian u|)lift passes directly into the synclinorium of the Illinois coal 

 field. It is to be observed in connection with these discrepancies that 

 they involve no great difference of di]^ and little shortening. 



It is not always possible to determine the precise age of individual 

 folds where Carboniferous rocks are not involved in them. In making 

 such determinations this principle has been followed ; if a fold in the 

 Carboniferous rocks extends directly into a similar fold of the under- 

 lying rocks so that both form consistent parts of a whole, it is considered 

 that the fold in the older rocks also is of Carboniferous age. While this 

 method may occasionally lead to mistakes, it is believed to be sound on 

 the whole. The fact that the axes worked out in this way form a con- 

 sistent plan is the best proof that the procedure is correct. 



PERIODS OF FOLDING 



The subject of this discussion is naturally the deformation at the end 

 of the Paleozoic which j^roduced the special characters of the Appalachian 

 system. Deformation of the Appalachian rocks was by no means limited 

 to that period, however, but took place at various times before, during, 

 and after the Paleozoic. Xone of these upheavals, however, compare in 

 intensity and geographic range with that which ended the Paleozoic ex- 

 cept the movements which closed the Triassic. The latter deformation 

 was of a totally different kind from that of the Aj^palachian revolution. 



The various earth movements fall into two classes whose results differ 

 enormously. First, because most common, are the broad epeirogenic 

 uplifts which caused oscillations of the crust with slight domes and 

 basins. These movements are recorded by overlap and hiatus in deposits, 

 without noticeable differences of dip. Such movements took place at a 

 great number of epochs recorded in the strata of the Paleozoic, Triassic, 

 Cretaceous, Tertiary, and Quaternary periods, and are doubtless going 

 on at the present day. 



The second class consists of the horizontal movements of the crust, 

 which folded the rocks laterally and pushed them up into long, high 

 arches separated by narrow troughs. The chief of these movements 

 closed the Paleozoic and is called the Appalachian revolution; it con- 

 sisted of a double movement in Carboniferous time, the second being far 

 the greater of the two. A similar movement took place in eastern ^ew 



