610 SIK J. W. DAWSON ON BFRKOWS AND TRACKS OF 



foration in the centre, often containing a little vegetable matter. 

 They were thus described * : — 



" These little bodies are evidently clay concretions formed around 

 vegetable fibres, and hardened by a small percentage of calcium 

 carbonate, since when treated with hydrochloric acid they effervesce 

 feebly and become disintegrated. They probably originate in the 

 molecular aggregation of the calcareous matter in the clay around 

 any foreign body included in it. They are about half-an-inch in 

 diameter, and the largest may have been two inches in length ; with 

 rounded ends. When broken, they show a small central canal con- 

 taining a little sand and strips of epidermal tissue, the remains of a 

 root or stem. One shows three branches, apparently proceeding in a 

 verticillate manner from a central stem. In the centre, the light 

 reddish-brown colour of the clay has assumed a greenish hue, owing 

 to deoxidation of the peroxide of iron by decay of the vegetable 

 nucleus." 



On comparison of these recent concretions with the Potsdam 

 cylinders, it becomes apparent that they resemble each other very 

 closely in form and structure, and that the older cylinders may have 

 been formed in a similar manner, though on a gigantic scale. In 

 confirmation of this view, it may be mentioned that in the Pleisto- 

 cene clays of Green's Creek, on the Ottawa, cylindrical concretions 

 surround twigs of poplar, which have been imbedded in the clay, 

 and that in the Permian Sandstones of Prince-Edward Island 

 ferruginous matter has cemented the sand into cylindrical concretions 

 around stems of Calamites. This view as to the origin of the Pots- 

 dam cylinders is further confirmed by the rounded ends of some of 

 them, and by the conformity of the internal concentric structure to 

 this rounding. One of the smaller specimens in the Peter-Eedpath 

 Museum shows this peculiarity very well. 



In the case of the Potsdam concretions, the nucleus of the con- 

 cretion must have been an erect stem of some kind, possibly a Chorda- 

 like Alga. So far as appears, this central stem must have been very 

 slender, but no distinct traces of it have yet been observed. Per- 

 haps the most remarkable fact in the case is that these cylindrical 

 bodies are sometimes several feet in length, and pass through more 

 than one bed of the sandstone. Another peculiarity is the presence 

 is some of them of irregularly rounded cavities, apparently indicating 

 the presence of bodies either concretionary or organic which have 

 been removed by solution or decay. These, however, are very rare. 



§ VI. Combinations of Woem-teacks with Kipple-maeks and 

 Sheinkage-ceacks. (Pigs. 14 & 15.) 



Pig. 14 shows a rippled surface in Potsdam Sandstone with marks 

 of worms or molluscs, arranged in the hollows of the ripples. The 

 marks are simple trails, of that curious circular or chain-like form 

 sometimes observed, and seem to have been made by animals creeping 

 in the furrows between the ridges of the ripple-marks. 



* ' Canadian Eecord of Science,' vol. iii. No. 5, January 1889, pp. 292-294. 



