STRUCTURAL GEOLOGY. 



489 



laterally into sandstone, and this into shale or limestone. It is indeed 

 rather more remarkable that the sedimentary strata should be as regular 

 and persistent as they are, than that they sometimes grade into one 

 another. 



Special markings. — The rhythmical action of waves gives rise to 

 undulatory lodgment, known as ripple-7narks (Fig. 324). They are 

 usually not the direct product of the surface- waves, since they are much 

 too small. They are produced mainly by the vibratory movement of 

 the undertow, but they apparently result from various other phases of 

 vibratory agitation of the bottom waters. They are sometimes made 

 by streams and stream-like currents. Ripple-marks are apparently 

 preserved indefinitely under proper circumstances. They are sometimes 

 found, for example, on very ancient quart zites. Ripples are also made 

 by wind (p. 37). Ripple-marks are usually only an inch or two from 

 crest to crest, but in rare instances they attain much greater size. 

 Examples of ripple-marks 30 feet across are known. ^ Occasional ridges 



Fig. 369. — Mud-cracks in Brunswick Shale, N. J. (Kiimmel.) 



and depressions of much greater dimensions are produced which are 

 attributable to the formation of successive bars, or to the building of 

 wave-cusps.^ Rill-marks are not infrequently produced by the under- 



^ Gilbert. Bull. Geol. Soc. Am., Vol. X, pp. 135-140, 1898. 

 2 Branner. Jour, of Geol, Vol. VIII, pp. 481-484, 1900. 



