304 AUGUST F. FOERSTE 



Formerly, pebbles were regarded as unequivocal evidences of 

 the proximity of a shore line, but even these may be formed below 

 water-level. This is true especially of the pebbles found in the 

 Ordovician strata of Ohio, Indiana, and Kentucky, since these 

 usually occur only in ripple-marked layers, or in lateral extensions 

 of these layers. The same causes which gave rise to the ripples 

 may have given rise to the pebbles. 



These causes apparently include a more or less rapid flow of 

 water. The ripple-marked layers usually consist of more or less 

 f ragmen tal detrital or organic material, frequently in much greater 

 quantity and of coarser grain than in the immediately overlying 

 and underlying layers, as though freed from the accompanying 

 calcareous and argillaceous muds by repeated rewashings of the 

 materials constituting the ripple-marked layers. These muds 

 either were washed to more distant areas or were, in part, held in 

 suspension in the overlying waters for a short time. The ripple- 

 marked layers frequently show evidences of cross-bedding, espe- 

 cially immediately beneath the crests of the ripples, thus also 

 suggesting current action. The larger pebbles, a foot or more in 

 diameter and only an inch or two in thickness, may easily have 

 been formed by currents dissecting a more or less fine-grained 

 stratum, and leaving remnants of the latter more or less imbedded 

 in the current-washed material farther on. In limestone layers 

 not exceeding four inches in thickness, even directly beneath the 

 crests of the ripples, the larger pebbles scarcely could incline very 

 much. The source of the pebbles readily could have been some 

 formerly existent layer located less than a foot above the present 

 level of the pebble. The finer muds of the intervening section 

 could have been washed away, and the coarser material retained 

 to form the major part of the ripple-marked layer, in the upper 

 surface of which the limestone pebbles are imbedded. 



Especial attention should be called to the fact that, even where 

 the ripple-marked layers are most abundant, many of the inter- 

 mediate limestone layers may show no trace of ripples. Hence, the 

 frequent absence of ripple-marks needs explanation fully as much 

 as their locally more or less frequent presence. 



