of Bitrling'ton, Iowa, and its Vicinity. 213 



often suddenly and independent of stratification, yet also 

 at times dependent upon it ; and finally passes gradually 

 upward into a yellowish, fine-grained sandstone. The 

 lower parts, and particularly where it assumes a deep blue 

 color, readily disentegrate into a soft, clayey material 

 upon exposure to the atmosphere, but the upper parts are 

 often solid enough to resist the action of the atmosphere 

 and frost, and at one point it is quite calcareous and solid. 

 This bed has formerly been divided into a lower bed of 

 " blue, shaly clay " and an upper bed of " ash-colored grit- 

 stones," but as the fossils found in the lower part are 

 recognized in the upper part, and as no line of separation 

 can be observed, the whole is considered here as one bed. 



It is apparently composed of a blue, argillaceous mate- 

 rial and fine yellow sand, with a limited amount of car- 

 bonate of lime as a cement. Where the argillaceous 

 material predominates, the mass is of a blue color and 

 rather soft. When the yellow sand and blue clay are 

 more nearly equal, it assumes a greenish color and be- 

 comes harder. And when, as in the upper part, the yellow 

 sand is in excess, and contains a greater amount of car- 

 bonate of lime, it becomes a yellowish sandstone. The 

 changes of color, however, are not always entirely due to 

 the mechanical mixture of the sand and clay. The fossils, 

 with few exceptions, are in the form of casts ; in the 

 upper part they are generally abundant, but in the lower 

 part they are rare, and are sometimes preserved in sul- 

 phuret of iron, the cavities which the fossils once occu- 

 pied having been filled with this substance. In these 

 parts, also, crystalline masses of bi-sulphuret of iron are 

 often found. 



Near one point, where this bed measures sixty feet above 

 the level of the river, it has been bored for water to the 

 depth of seventy feet below this level, without any appar- 

 ent change in the character of the bed, and leaving an 



