342 W. I. ROBINSON 



mode of origin. The linear terrace folds result from long continued 

 gentle movements, and the monoclines produced by faulting are 

 the expression of deep-seated faults which have decreased in effect 

 toward the surface until, in the upper layers, there is distortion of 

 the beds without actual rupture. 



DEFINITIONS AND CONCLUSIONS 



Domes or quaquaversal folds. — A. Inconspicuous folds usually 

 small formed in undisturbed areas; the force secondary and small, 

 vertical. The force may be {a) a chemical action or crystallization 

 (salt domes), {h) the vertical component of lateral thrusts set up 

 within the sediments, still soft, by the sudden accumulation of 

 heavy sediments shoreward," or (c) the unequal contraction during 

 induration of sediments of differing compressibility. No topo- 

 graphic expression. m 



B. Conspicuous folds often large, the force coming from move- 

 ments of magma. Domed mountains above bathyhths, laccoliths, 

 lava plugs. Topographically striking. 



Radial linear folds. — Small, radial, about basins of slight negative 

 tendency; become apparent through drill records; dip of limbs well 

 within the limit of initial dip; distinguished from folds due to 

 unequal contraction during induration only by radial arrangement 

 and persistence of fold to the basin floor contemporaneous with 

 subsidence. Dominating force vertical and downward with 

 resultants tangential in concentric areas giving compression effect 

 parallel to periphery of basin. 



Other explanations: unequal subsidence which would be more 

 likely to cause faulting; unequal distribution or unequal solution 

 in evenly distributed, easily soluble material such as salt. 



Concentric terrace folds. — About basins of negative tendency. 

 Caused by a failure of concentric areas, or by differences in compres- 

 sibility of sediments. Simulate terraces due to thinning of sediments 

 toward center of basin. Known only from comparison of deep 

 borings. No topographic expression. 



'See M. Albertson, Mining and Metallurgy, No. 170 (February, 1921), p. 38. 



