REPORT OF THE DIRECTOR I916 203 



that he had detected on the same sandstone stratum in outcrops 

 twenty miles apart, striations that ran in the same direction and 

 suggested to him a uniform cause efficient over this entire stretch 

 of strand or sea bottom. I regret very much not being able to verify 

 so important an intimation. " Thus it appears," says Hall, " that, 

 whatever may have been the cause, it operated very uniformly over 

 large surfaces." 



It is important for me to premise, in seeking the interpretation of 

 these phenomena, that, in my experience, ripple marks in the Portage 

 strata are of rather rare association with these other strand marks. 

 If this statement is even approximately true, then the rhythmic fac- 

 tors producing ripple marks are measurably excluded from the 

 formation of the strand marks under consideration. Doctor Kindle's 

 perspicacious and faithful analysis of the causes to which ripple 

 marks are due, finds, as we have intimated, that whether laid under 

 water or under air, a sandy medium is essential ; that ripple marks 

 are of two kinds, symmetrical and asymmetrical ; that the former 

 are due wholly to oscillatory action of the water and occur, in his 

 conclusion, entirely in fresh-water deposits, that is, in water bodies 

 presumably free of tidal currents ; while the other or asymmetrical 

 type characterizes the salt water ripples. Disturbance in the cur- 

 rents, interfering waves or tidal flows, obstructions to regular flow, 

 projections of rocky points between sandy beaches and over seeth- 

 ing bottoms, are all influential in modifying the form of the ripple 

 ridge and furrow, and the amplitude of the sand wave is in a definite 

 though not determined relation to the force of the tidal ebb. There- 

 fore, in the opinion of Doctor Kindle, which has been supported by 

 the observations of others, principally those of Dr G. K. Gilbert, 

 the ripple mark may be of any reasonable amplitude, often attaining 

 a width of many feet. 



The objects which we have under direct consideration, divide 

 themselves into two groups ; those which seem readily to explain 

 themselves and those which obviously do not. 



To the former we may reckon the characteristic " mud flows," 

 as shown on our plates 7-14. These, it will be observed, are 

 obviously the markings made by rills following the fall of the tide, 

 or possibly the retreat of heavy storm waves on a strand' of low 

 pitch. These phenomena have various expressions and are often 

 accompanied by collateral contemporary markings showing the 

 dragging, scratching or otherwise indenting of the surface, pre- 

 sumably by objects dragged down by the ebb and following the 



