MODE OF PRESERVATION 209 



marked degree from portions of proboscidean and ungulate skeletons 

 which have been received from apparently younger gravels at Le Mars, 

 Rock Rapids, and a few other points in northwestern Iowa. In many 

 cases the Aftonian sand and gravel are cemented into a hard conglomerate 

 around the fossils, forming an indurated siliceous shell, ranging up to 

 half an inch in thickness, and adhering more or less firmly to the inclosed 

 skeletal remains. The deposition of silica, which altered the composition 

 and hardness of the fossil bones and teeth, affected the inclosing matrix 

 of sand and gravel for a short distance outside the surface of contact. 



In a majority of the gravel pits the fossils are stained more or less 

 with metallic oxides. Ferric oxide is one of the products of chemical 

 changes caused by the weathering of pebbles carrying iron-bearing min- 

 erals, and there are few Aftonian exposures that do not show streaks and 

 pockets of manganese dioxide. The bones and teeth where these oxides 

 occur are colored in various shades, ranging from light to very dark 

 brown. Where clean quartz sand predominates, the fossils, while becom- 

 ing silicified as completely as in the other cases, retain their original 

 whitish color. The Elliott pit at Turin contains largely coarse, sharp 

 quartz sand, and stands almost alone in furnishing specimens conspicu- 

 ously light in color, free from metallic stain. The same fauna, however, 

 occurs in all the pits of true Aftonian — the same horses, proboscideans, 

 ground sloths, and camels, and the skeletal remains — whether from Rock- 

 port, Missouri; Sioux Falls, South Dakota, or from points between — all 

 show the same mode of preservation, all, especially from the siliceous 

 phase of the formation, are fossilized in about the same way. 



Carnivores 



One of the remarkable features of the Aftonian fauna, so far as the 

 very meager knowledge of it has yet progressed, is the practical absence 

 of Carnivores. At the time the first paper was written no remains of 

 carnivorous mammals had been received from undoubted Aftonian de- 

 posits, and the work of the past two years has brought to light only one 

 specimen to represent the flesh eaters, the right ramus of a large bear 

 (plate 18). The teeth remaining in the jaw are the canine, fourth 

 premolar, and first molar, and all are very much worn. The second and 

 third molars have fallen out since the death of the individual, possibly 

 since it was collected by the workmen in the sand pit. The canine is very 

 large, elliptical in cross-section, and shows evidence of great wear on the 

 tip and anterior edge, and this wear has reduced it to a mere blunt stump 

 of what it was when showing its full length. Evidently the animal had 

 reached a ripe old age. The dimensions are : greatest length of ramus, 



