OSCILLATORY CHARACTER OF CONTINENTAL SEAS 335 



ville dome (see page 307) and the Yellville to Osagian formations in the 

 larger embayments of the southern shore of Ozarkia. The Yellville es- 

 pecially offers interesting features (see pages 421 and 667) in that the 

 distribution of the successive members or zones of tlie formation is not at 

 all the same in the several embayments. 



Frequency of alteration of submergent and emergent conditions in- 

 dicative of smallness of Paleozoic movements of land areas with respect 

 to sealevel. — The mere fact that the sea advanced many times in geologic 

 history over continental areas, and that the submergent stages alternated 

 with a like number of times when emergence of lands prevailed, argues 

 strongly for the smallness of the vertical element that is responsible for 

 the displacement of the strandline. I do not refer to the comparatively 

 limited "positive" areas, whose tendency has always been toward relative 

 uplift and which doubtless rose to considerable and, at times of unusual 

 diastrophic activity, perhaps to great altitudes, but to the broader "nega- 

 tive" areas which were commonly, or at least likely to be, inundated in 

 the submergent phases. Eegarding these negative areas only a slight 

 general displacement of the sealevel would effect the submergence or 

 emergence, as the case may have been, of wide stretches. 



Wide areas characterized by negative tendencies, like that between the 

 Appalachian folds on the east side and the Cincinnati axis on the west, 

 comprise nearly parallel bands having the essential anticlinal structure of 

 positive areas. Sometimes these have formed important, though rarely 

 complete, barriers, against which marine sediments overlapped from 

 either side. Eather generally, too, the location of "pools" of petroleum 

 and natural gas have been controlled by them. However, as tectonic fea- 

 tures, they have been subordinate to the negative troughs flanking them 

 and were dragged down with these and included in broad geosynclines. 

 Tn the case mentioned there are many minor folds besides at least two 

 that were important at times. One of the latter, the Carter axis (see 

 map, page 293), is a marked structural feature in eastern Kentucky, 

 having formed the western shore of several Paleozoic seas. The other is 

 the Helderbergian barrier, which limited the western extent of certain 

 Appalachian formations. The Appalachian Valley, with its subordinate 

 anticlines and troughs, and each of the latter with a different section, is 

 a more striking example of a compound syncline, but on account of its 

 complicated structure and stratigraphy it is less easily described. How- 

 ever, a fair idea may be gathered from the description of the Chambers- 

 burg limestone in southern Pennsylvania (pages 321 to 329) and from 

 the discussion of southern Appalachian formations in Part IT (images 

 412 and 543 to 571) and Part III. 



