NATURE OF SUPPOSED FOSSIL TRACKS. 489 



the impression; and thirdly, the unbroken continuity of the im- 

 pression throughout its entire length. It must be evident that there 

 are only two ways — both exceedingly improbable — by which these 

 impressions could by any possibility have been made by any animal, 

 whether crustacean or mollusk, or member of any other group. If 

 the impression be really a track, the animal must either have had, or 

 have been able to assume, the form of a complete sphere or cylinder 

 with ribbed sui'face, and it must have possessed svifl&cient internal 

 force to roll itself over and over throughout a length of many feet ; 

 or otherwise the creature must have moved forward by a series of 

 spasmodic jerks or jumps, alighting always in an exact line with the 

 end of the trail, so as to avoid the slightest overlap) or other break of 

 symmetry in the entire impression. Any other mode of progression 

 would unavoidably have effaced or smudged the transverse grooves 

 or ridges as the body of the animal passed over them. If formed by 

 a mollusk also, we might naturally expect to find the shell of the 

 creature (or at least casts of the shell) in the suri'ounding strata, 

 because, if the transverse ridges were formed by the creeping foot, 

 the beaded rim must be attributed to the aperture of the shell ; and 

 the latter, consequently, must have been of large dimensions, and the 

 shell itself of considerable weight and solidity, and thus not unlikely 

 to have become fossilized. The casts of gasteropod shells — Ophileta, 

 Pleurotomaria — but none to which these impressions can be attri- 

 buted, are not altogether absent from our Potsdam beds : and if these 

 have been preserved, why not others 1 There is also another point 

 which appears to be in complete opposition to the assumed track- 

 origin of these impressions. In places, two, or even three, of these 

 supposed tracks cross one another, but at the crossing points there is 

 no sign of disturbance or smearing, so to say, such as must inevitably 

 have occurred if one trail had been carried across another. As shown 

 especially in Sir William Logan's original figure, representing a group 

 of several "tracks" (Geol. of Canada, p. 107), the one impression 

 simply conceals or lies over the other at these points, as would happen 

 if two fucoid-fronds, or other similar bodies, were drifted together to 

 a sandy shore, and were there covered simultaneously with sediment. 

 In attributing these impressions to large fucoids, we encounter, on 

 the other hand, no real difiiculty. Many algse, it is well known, 

 present transverse furrows ; and a salient example of this character 

 may be seen in our Arthrophycus Harlani, so abundant in many of the 



