1911.] NATURAL SCIENCES OF PHILADELPHIA. 54o 



surfaces. On one occasion a caribou almost walked into camp 

 during the early morning and it traversed about two miles of the 

 flat to the west. Its foot-prints were deeply impressed into the 

 clay surface; on the rippled parts they were more than the depth 

 of the ripple-mark. The median digits were impressed below the 

 level of the trough of the ripple, and even the lateral digits sometimes 

 made a mark, especially where the foot had slipped slightly. The 

 clay was so firm, however, that the foot-prints did not exceed one 

 inch in depth at any place, although this deer is a heavy animal. 

 The impressions were very persistent, they continued to be dis- 

 tinctly outlined for days after they had been formed; and, indeed, 

 were quite distinguishable when we left the camp, about a week 

 after they had been impressed in the clay. They probably lasted 

 much longer, yet they were covered by the tide twice in each twenty- 

 four hours during the period of observation. If this water had 

 contained any considerable amount of sediment they might have 

 been covered and preserved. 



At another time one or two bears were roaming about on the 

 flats; they left a much less distinct impression, scarcely flattening 

 out the ripples in the clay, though in some cases their claws were 

 deeply impressed. This indicates the hard character of the clay, 

 for these animals have considerable weight. Being plantigrade, the 

 larger surface distributes the weight, so that the foot-prints were 

 not deeply impressed. Nevertheless, they could be traced after the 

 flats had been covered by several succeeding tides, and the claw 

 portion of the track was visible after 5 or 6 tides. 



Some dogs (temporarily abandoned by their owners, who had 

 gone out to the coast to fish during the summer) attached them- 

 selves to our camp; and their tracks, made when they were coming 

 in for their meals, were also found to last for two or three days. 

 Our own tracks were about as permanent, unless in the softened 

 clay, where they were of course more deeply impressed and there- 

 fore much more permanent. The tracks made by the birds only 

 affected the shallow layer of soft clay, perhaps about one-eighth of 

 an inch dee]), and they generally could not be seen after one tide 

 had covered them. In some of the unrippled places, however, the 

 bird tracks seemed to be covered by clay, and- were doubtless pre- 

 served until the surface was again eroded. Of course, if much 

 sediment were being deposited, they might be preserved permanently, 

 but here they were generally only temporary. 



These animal tracks, when deeply impressed into the clay surface, 



