CAMBRIAN AND ORDOVICIAN RIPPLE-MARKS 713 



former existence of shallow seas where corals are the dominant 

 fossils, and it seems an equally safe deduction to infer a sea of con- 

 siderable depth where stalked crinoids comprise the dominant 

 element in the fossil fauna as they do in the ripple-mark horizon 

 under consideration. The occurrence on the ripple-marks of 

 numerous crinoids in which the head remains joined to nearly 

 complete stems (Fig. 6) affords direct evidence that they lived 

 in water of sufficient depth to be seldom disturbed by wave action. 

 The well-known fragile character of the union of the individual 

 segments of a crinoid column and of the head and column could not 

 be expected to survive the bottom disturbance of ordinary wave 

 action. It is probable that the ripple-marks on which the crinoids 

 rest are the result of storm waves of unusual size whose oscillations 

 penetrated to a depth not affected by ordinary waves. It appears 

 from these considerations that both the physical and the faunal 

 evidence suggest for the ripple-marks in the Trenton limestone an 

 origin in water of greater depth than the ripple-marks in the Cam- 

 brian limestone. 



