ber 
274 FIELD CoLuMBIAN MusEuM—GEovocy, VoL. I. 
islands on which they have: stranded. In one instance part of a 
skeleton found imbedded in a stratum of blue clay which thinned out 
and was replaced by sandstone with pebbles at the base, indicates that 
the carcass was buried in.a mud-bank bordering a stream or water- 
current. The interruption of the vertebral column and the displace- 
ment of the ribs in one direction show that the stream was sufficient 
to carry away the missing part of the skeleton. 
The tendency toward a more shaly nature and the presence of 
carbonaceous matter in the upper measures indicate the return of 
shallows and the greater abundance of vegetable matter. This con- 
dition evidently culminated in the great influx of sand laden with 
deciduous leaves which marks the period represented by the Dakota 
sandstone. 
