206. BOTANICAL GAZETTE [SEPTEMBER 
prevents the development of the ultimate stage of the succession. 
Each year the bottom of the ditch fills up with silt which is removed 
to the bank the following spring. This forms a new soil for the 
entrance of ruderal species, and as a result the bank returns to a 
somewhat primitive condition. 
Another factor of importance here is the fact that during the winter 
the ditch is empty the greater part of the time. This condition is 
detrimental to the success of the species of the ultimate formation. 
The result is that along these irrigation ditches are found various 
stages of anomalous successions, which are checked repeatedly, or 
even turned back by the reversion to more primitive conditions. The 
successions are interrupted again and again, and this may account to 
a great extent for the variable character of the bank formation. 
° General discussion. 
After careful study it is at once apparent that all parts of the 
formation are not of the same age. The secondary successions 
throw much light upon the structure as well as upon the development 
of the formation. These exhibit rather well-marked stages. The 
first is a ruderal consocies; the second, a society of the formation; 
while the ultimate stage is a consocies of the formation. All of these 
successions lead undoubtedly to the Bouteloua formation. 
The successive deposition and erosion which has produ 
Great Plains does not differ markedly from that which may 
noted at the present time. The vegetation on new deposits of 
passes through the typical succession leading from the ruderal to 
the grass formation. Near the mountains the cutting back of the 
gullies results in the establishment of the thicket formation, which . 
preceded usually by the entrance of many of the secondary species 
of the thicket and grass formation. The pine formation is usually 
mixed with the thicket formation. When this cutting back of - 
gullies exposes rocks of a sufficient degree of stability, the primitive 
lichen formation precedes all others. 
In many places gullies are cut back which are reclaimed at we 
by the grass formation. The thicket and pine formations do es 
enter. In these places the Andropogon scoparius consocies usually 
becomes established. 
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