Stockdale: Stylolites 



17 



occupying the depression (see Fig. 8). Pebbles bearing this 

 type of indentation are usually not distorted or fractured. 

 Some impressions, however, appear to be a gradation between, 

 or combination of, the two above-mentioned types. The prin- 

 cipal theories of their origin are two in number: (1) that 



Fig. 7. — Fractured type of impressed pebble from the Karbon- 

 formation. Original in Marburg Museum. (From Kay- 

 ser's Lehrbuch der Geologic.) 



Fig. 8. — Solution type of impressed pebble from the Nagel- 

 fluh. (From Kayser's Lehrbuch der Geologic.) 



they are a result of a mechanical force; (2) that they are a 

 result of chemical action — the solution of the one pebble at 

 the point of contact of the other. Some investigators believe 

 the feature to have resulted while the pebbles were in a 

 plastic, or semi-plastic state.- 



- Early suggestions of this "plastic theory" were made by Hitchcock: Geolojo' of 

 Vermont, I, p. 28 ; Proc. Bost. Soc. Nat. Hist., VII, pp. 209, 353 ; XVIII, p. 97 : XV, 

 p. 1 ; XX, p. 313 ; Amer. Jour. Sci., 2d Series, XXXI, p. 372. 



