PORTAGE GROUP. 
237 
103. 
Cast of Mud-furrow, with Shells.*—Natural size. 
This cast has upon its surface the marks of numerous shells, all of single valves, which 
appear to have been quietly floated along over the even surface, till they lodged in the de¬ 
pression from which they could not be removed. The succeeding deposition covered them, 
and they became attached to it, and it now appears as a cast in relief. Upon the lower surface 
of the layer from which the ridge rises, there are no shells, though they are quite numerous 
on the ridge. 
Many other interesting illustrations of similar kinds could be given, but these appear suffi¬ 
cient for the occasion, and to show their origin, which proves at least some interesting facts 
in relation to this period. 
Localities. — Almost every ravine and stream upon the elevation which rises to the south 
from the Hamilton group, exposes the rocks of the Portage group in greater or less perfection. 
Along the banks of Cayuga and Seneca lakes, towards the southern extremity, these rocks 
form cliffs, often of considerable height, which, from the alternating hard and soft layers and 
the numerous vertical joints, present the appearance of solid walls of masonry in distinct and 
regular courses. Isolated masses, like some huge column, are often seen standing out in bold 
relief from the line of the cliff, being the remains of a previously exposed surface which has 
crumbled away. 
* It must be borne in mind that the upper surface, as represented, is the lower in nature. 
