68 BOTANICAL GAZETTE [AUGUST 
floating mat. The thickness of purely allochthonous (transported 
to the place of occurrence) layers of peat, therefore, is far less 
extensive than might be assumed, partly also because submerged 
and amphibious plant populations can take root in depths varying 
from ro to 15 feet (3-4.5 m.) and accumulate as peat in situ. The 
macerated type of peat is nevertheless preeminent, arid it varies 
least in character under conditions which give rise to water colored 
brown from the presence of suspended and dissolved organic débris. 
On account of the decrease in light and heat available, and the 
consequent absence of submersed plant communities, the filling 
of the depression is chiefly from vegetation units bordering the 
basin. The colloidal and doppleritic types of peat, on the other 
hand, make clear another set of conditions; they appear to indicate 
a higher calcium carbonate content of the waters at the time of 
their formation, and stimulating environmental conditions of tem- 
perature and light, in which the growth of aquatic vegetation 
units and planktonic organisms probably reached unprecedented 
proportions. There are reasons for concluding that the colloidal 
and doppleritic types of peat may represent another kind of evidence 
of climatic fluctuations. In the deposit near Fremont, Indiana 
(fig. 2), for example, colloidal material alternates with layers of 
macerated and “acidic” plant remains. The formation of colloidal 
material, therefore, may correspond in time with conditions of 
drought, when the lake or pond waters were concentrated by evapo- 
ration and became alkaline as concentration progressed. It is quite 
probable that the finer calcareous material in the drift had been 
removed by leaching, and produced variations in the chemical 
composition of the lake and ground waters. The calcium carbonate 
content when separating in the open water in a finely divided state 
must have become mingled with the plant débris so as to form a 
flocculation product and in places an end product of plant disinte- 
gration combined with lime. The climatic changes which brought 
about this condition may not have been sudden or excessive, but 
probably were oscillations of moderate intensity, whose cumulative 
effects were felt during that period of time. 
The rate of building up a peat deposit in lakes or ponds appears 
to increase considerably as a plant population such as that formed 
by sedges pushes out from the shores, becomes nearly or quite 
