114 P. E. Raymond — Seaside Notes. 



Planolites superbus "Walcott may or may not be 

 organic. It lias somewhat the appearance of the cast of 

 a tunnel parallel to the bedding and if so, may have been 

 made by a gastropod, a crustacean, or possibly, an 

 annelid. 



Only one of these Pre-Cambrian trails presents any 

 appearance which would seem to entitle it to be called the 

 burrow of a worm. The others are more probably either 

 of a gastropodean origin, or are entirely inorganic. 



The Preservation of Trails. 



After studying the trails on the beaches it becomes evi- 

 dent that to allow their preservation somewhat unusual 

 conditions must prevail. Trails made during the ebbing 

 of the tide are completely obliterated in the flow, 

 and those made in shallow Water are destroyed by the 

 motions of currents and waves. To insure preservation, 

 it appears to be necessary that the mud shall contain suf- 

 ficient cement, in the way of calcium carbonate, oxides of 

 iron, hydrous silica, or finely divided clay, to consolidate 

 the surface quickly. An example of the latter process 

 was noted recently. A fresh bird-track was noted in mud 

 which was not yet dry at noon on Monday. Tuesday 

 noon it was dry and firm. Tuesday night there was a 

 hard rain, and Wednesday noon the track was still visible, 

 covered with about two inches of water and partially 

 filled with fine mud. It had not lost its clear outlines and 

 enough cementation had taken place during the drying 

 process to allow preservation. 



Although it is possible that such action may take place 

 between tides, as indicated by Agassiz's oft-quoted obser- 

 vations on the calcareous mud of Florida, it does not 

 occur in ordinary sediments. The flood plains of deltas, 

 and the playa deposits of arid regions present ideal con- 

 ditions, but such localities are not inhabited by marine 

 animals. To preserve the trails of the latter requires 

 the postulation of some such processes as are outlined 

 above in connection with the jelly-fishes, and the abun- 

 dance of trails in formations of all ages would seem to 

 indicate the presence of marine playas throughout 

 geologic time. 



Geological Museum, 

 Cambridge, Mass. 



