204 



ROCHESTER ACADEMY OF SCIENCE. 



In the Genesee gorge, D. archhnedes occurs through the upper 

 30 feet of the Medina ; A, al/eghaniensis is restricted to the lower 14 

 feet of this zone. The rock is a sandstone. The upper portion is 

 argillaceous and homogeneous. The bedding planes are not pro- 



Fig. I. — Diagram showing the general mode of formation of Dcsdahis. 



The upper dotted line represents the lower side of the burrow in its first position ; the lower 

 dotted line, the upper side of the cylinder, or burrow, in its last position ; d, e, z, the strips, or pack- 

 ings made as the burrow was shifted ; /, the thin edge ; E, lower portion of the last two packings 

 and the cylinder, enlarged, showing the caplike terminations of the strips, one fitting up into 

 another and the last ensheathing the end of the cylinder. 



nounced and weathering carries the ledge back with an even face. In 

 the lower portion, the rock is purer sandstone and the layers are 

 separated by seams of shale which are more or less arenaceous. This 



