FOLDS RESULTING FROM VERTICALLY ACTING FORCES 337 



DOMES 



These are the conspicuous folds resulting from an upward 

 acting force, or from a set of forces whose resultant is active upward ; 

 as such they have been described for many years. They are often 

 found to be genetically related to a bathylith, laccolith, or lava 

 plug, but sometimes they are merely the locus of chemical action 

 or of recrystalhzation which results in a great increase of volume 

 within a limited area, as may be the case in the salt domes of the 

 Gulf states whose origin is imperfectly understood. Less fre- 

 quently, sHght domes arise from the slumping of soft sediments. 

 This is a phenomenon noted at times in present-day accumula- 

 tions. Others are found in areas of different compressibility in 

 the same layers of rock, especially near shore lines. How much 

 difference in elevation the subsequent rock pressure might bring 

 about upon such areas of unequal compressibility, cannot be es- 

 timated with the incomplete data at hand, but this may prove to 

 be an important factor. In size, domes vary from great groups of 

 mountains to small inconspicuous rises of ground a few acres in 

 area. 



RADIAL LINEAR FOLDS 



Small folds are found near the periphery of large basin structures 

 with their axes pointing toward the center of the basin. They are 

 so broad and shallow that they are seldom recognized except 

 when the section is plotted with vertical exaggeration. The dip 

 of the limbs of these folds as they are found in the Michigan basin 

 is well within the limit of initial dip of sediments, and the pitch 

 of the axes is roughly parallel with the dip of the series as a whole. 

 These folds, if known only for a single formation, would be con- 

 sidered to be due to peculiar conditions of deposition, but from 

 the fact that they persist through many formations, and appear 

 at points on every quadrant of the basin, they are obviously struc- 

 tures formed after consolidation. The uniformity of their distribu- 

 tion and their dimensions favor the idea, which was first proposed 

 by Professor I. C. Russell,^ that they are a result of the subsidence 

 of a central area resulting in a crinkling of the peripheral portion 

 just as an unfolded filter paper crinkles when pressed into a funnel. 



• Mich. Geol. Stirv. Pub. 12, Geol. Ser. 9, p. 207 



