162 PROCEEDINGS OP TIIIC ALBANY MEETING 



currents of constant direction is discussed ; they are contrasted with current- 

 marks and sand-waves. Field evidence is given to the effect that the only 

 current to which the formation of the Ordovician and Silurian "giant ripples" 

 can be ascribed is the tidal current. 



The horizontal distribution of these ripples in the Silurian of the Cincinnati 

 anticline is contrasted with that of the Ordovician of the same area, and the 

 peculiar geographical conditions necessary for the production of each are dis- 

 cussed. The rhythmic recurrence of the different sedimentary units, char- 

 acteristic for the Ordovician of this region, is explained and the depth of the 

 corresponding ocean determined to be less than fifteen fathoms. 



Discussion 



Prof. A. W. Gkabau : Two points have always puzzled me about fossil ripple- 

 marks. The first: How can beach ripples and those made by tidal currents 

 and exposed between tides be preserved when every succeeding tide or wave 

 will obliterate the older series and make a new one? The other is this: How 

 can we understand the total absence of fragments of organic remains in such 

 beautifully ripple-marked sandstones as those of the Berea and Bedford of 

 Ohio if these formations are marine? Rippled beach sands, and especially 

 rippled sands of deeper water in the modern ocean, always contain some 

 vestiges of organic remains. 



Prof. G. H. Chadwick : In the Mohawk Valley, just after one of its largest 

 floods, there was observed a remarkable series of gigantic current ripples in 

 mud, whose crests, perhaps 15 or 20 feet apart, showed above the subsiding 

 waters. It should not be forgotten that the first use of the name "giant 

 ripples," by Gilbert, had reference chiefly to a wholly different phenomenon, 

 the isolated "beach-crests" of the Medina sandstone, later more fully explained 

 by Fairchild. 



Further remarks were made by Prof. C. H. Brown, with reply by the 

 author. 



At 10 o'clock the reading of miscellaneous papers was discontinued 

 and place given for the Symposium on the Interpretation of Sedimentary 

 J{()cks, under the titles which are given below: 



THE PROBLEMS STATED 

 BY A. W. GRABAU 



SWNTFWANCE OF SEDIMENTARY RHYTHM 

 BY JOSEPH BARRELL 



DIAGNOSTIC (JHARAOTERISTICS OF MARINE VLASTICS 

 BY E. M. KINDLE 



CHARACTERISTICS OF CONTINENTAL CLASTICS AND CHEMICAL DEPOSITS 

 BY ELIOT BLACKWELDER 



