200 NEW YORK STATE MUSEUM 



sandy shales, for when the shales are predominantly of clay, no such 

 marks seem to have been made. 1 This is a fact and an observation 

 which is in accordance with the interesting propositions recently 

 set forth by Kindle, 2 who finds that the muddy bottom or muddy 

 shore in which the sand is not predominant, is not competent to take 

 such current marks as the commonly known ripple marks. The 

 phenomena under consideration are not invariably those which can 

 be interpreted by any action of moving water alone, so far as known 

 to the writer, and it is necessary, therefore, to supplement the pre- 

 vious condition by stating that the shales of the Portage formation 

 are prevailingly sandy, and in the upper measures of this forma- 

 tion, wherein these phenomena are most conspicuously shown, are 

 quite inevitably sandy and therefore competent tp take and preserve 

 impressions that have been made by water in motion. 



As a preliminary necessary to the understanding of this paper, 

 reference is made at once to the accompanying illustrations. An 

 effort has been here made to present these strand surfaces in their 

 normal position and therefore counterparts in plaster of paris have 

 been photographed to place alongside the pictures of the original 

 rock specimens. If the observer's eye is quick to grasp these con- 

 trasting reliefs and intaglios, part and counterpart will help to 

 remove risk from deceptive appearances. I say deceptive appear- 

 ances, because even Professor Hall, in pointing out the fact that 

 these marked sandstones occur with their relief surfaces downward 

 in the strata, nevertheless so far forgot his statement as to illustrate 

 one of the commonest varieties as a " cast of flowing mud," and 

 such " mud flows " have been repeatedly referred to by geologists, 

 as though their convex relief were the true one. " Mud flows " are 

 in reality the natural casts of deeply marked strand slopes rilled and 

 eddied by the ebbing tide. 



In order, now, to present practically all the outstanding data 

 regarding the particular phenomena with which we have to deal, 

 I shall first quote from Professor Hall's very lucid description. 



Casts of Mud Furrows and Striae 



I have applied this term to certain appearances upon the under side of the 

 strata of sandstone, or flagstones, which are numerous and extensive in the 



1 Markings similar to some of those here noticed occur in great variety in 

 other rock media and in many formations. Notice is incidentally taken here 

 in the illustrations of such records on certain Silurian argillaceous limestones. 



2 E. M. Kindle. Recent and Fossil Ripple-mark. Mus. Bui. 25, p. 4, 

 Canada Geological Surv. 1917.; 



