272 F. W. SARDESON A CORROSION CONGLOMERATE 



most acceptably spoken of as worm-burrows, but which are more famil- 

 iarly called "fucoids." An abundant species of them here has been 

 named and referred by E. 0. Ulrich to the fossil sponges." These fossils 

 are twig-shaped, from 5 to 10 millimeters wide, more or less branched, 

 and are bluntly rounded at the ends. Their surfaces have organic sym- 

 metry, but internally they are filled with heterogeneous concentrate of 

 shell fragments, or whatever kind of coarse grains the surrounding ma- 

 trix contains. These '^fucoids" lie horizontally in the shales, but ofteii 

 bend conformably to the surfaces of shells or pebbles. I think they were 

 most probably the root-like hold-fasts of sea-weeds, the hollows or in- 

 teriors of which were later used by robber-worms as holes or burrows. 

 The hold-fasts were, of course, attached to the sea-floor, and in many 

 cases, if not as a rule, they were l)uried under partially consolidated sedi- 

 ment before being inliabited and consequently filled with the refuse 

 plunder of robber-wonns. They are even frequently associated with 

 undoubted fucoidal imprints that have the same size and profile as the 

 Camarodadia, but are flat. Those may belong to the same organic spe- 

 cies, but were not filled out with sand or other coarse debris. There are 

 also "fucoids" besides Camarodadia rugosa. The Licrophycus otta- 

 waense Bill., occurs in the ])ed (number 6) here and might be interpreted 

 as worm-burrows. Xone of these "fucoids" nor any worm-burrows pene- 

 trate the shales as if the sea-bottom had had more than a thin covering 

 of unconsolidated mud at any time. 



THE WORK OF ^"^EA-WEEDS 



The corrosion of lenses and nodules of limestone into conglomerate 

 may be explained as the consequence of slow deposition of sediment and 

 the action of sea-weeds in pulling up the sea-bottom. The corrosion of 

 the exposed surface of limestones has already been explained. To ac- 

 count for flat conglomeratic pebbles of limestone which are corroded on 

 both sides, they must further be supposed to have been turned over by 

 some force. Flat pebbles, a foot \\ade, would require a considerable force 

 to turn them over. The force might have come through sea-weeds. The 

 '^fucoids,^' already described in the preceding paragraph, indicate the ex- 

 istence of large kelp-like sea-weeds, which when anchored to calcareous 

 lumps on the sea-bottom and drawn by sea current might readily drag 

 them or turn them over. Wliatever the force that turned the flat pebbles 

 over, it also occasionally left one as now found standing edgewise. 



The reason for ascribing this conglomerate to the disturbance by sea- 



" Camarocla<iia rugosa Ulr. : Final report Geological and Natural History Survey of 

 Minnesota, vol. 3, part 2, 1897, p. xcv. 



