MICHIGAN ACADEMY OF SCIENCE. 
109 
is only in closed depressions of small extent that certain evidence may usually 
be found. 
The evidence of climatic conditions, as expressed in inter-glacial peat 
deposits and soil beds, is, of course, well-known, and needs no elaboration 
here. Such beds are indications not only of the retreat of the ice for pro- 
longed periods, but are definite records of their temperature and the humidity 
of the climate of the times. By much the same means that periods of drought 
and rainfall are indicated by the varying types of plant in the beds of a peat 
deposit, changes in drainage, due to various causes, are recorded. These 
changes may be due to the growth of the peat itself, and probably this is 
the more general cause, but the obstruction of outlets by drift material, 
by beavers, or in other ways, all produce the same effects. 
It seems probable that, in many cases, such obstructions as result from 
the growth of the peat, may be correlated with drought periods, since in 
times of low water-level in the peat, such plants as the shrubs, trees and 
Sphagnum make rapid growth, cover new territory, and may invade the 
sides and even the bottom of the outlets, clogging them, or even obliterating 
them entirely, so that when normal rainfall appears again, the water does 
not find outlet at so low a level as formerly and a renewed stage of very wet 
conditions prevails for a longer or shorter time and is recorded by layers of 
plant remains showing the changes. 
On the other hand, peat deposits of as great a thickness as 20 feet are 
known, which show, in a series of carefully taken samples, no marked varia- 
tion in structure or in the type of plants building the peat; this may be 
interpreted as indicating conditions uniform with those of the present for 
the entire period. 
Under certain conditions, the type of deposits under consideration may 
give records of movements of the crust of the earth, and, in some cases, 
form the most exact measure which can be obtained of the rate at which 
such movements are progressing. 
A number of writers have called attention to deposits of peat at various 
stations along the Atlantic coast, which are now below the high tide level. 
During the summer of 1907, the writer had an opportunity to examine several 
such deposits at widely separated points along the coast; some of these were 
in salt marshes, while others, subjected regularly to tidal overflow, were 
covered by water that was nearly or quite fresh. 
In the case of one salt marsh peat deposit, on the shore, near New Haven, 
Connecticut, the bottom of the deposit was a soil-bed with quantities of 
tree roots, stumps, etc., with a gravelly till below. The soil was nearly 
black, rich in humus, and was clearly similar to that usually found in tree- 
covered swamps of the present day, while the trees represented were swamp 
species of broad-leaved types. This layer was conspicuously darker than 
the peat above. Near the surface of the soil layer, shrub debris was abun- 
dant, and slightly above the root-crowns of the tree-stumps, these were 
replaced by an increasing quantity of the leaves and stems of sedges and 
other grass-like plants, above which for three or four feet such plants con- 
stituted the bulk of the deposit, forming a very characteristic, fibrous, brown 
peat, which was practically a turf, penetrated vertically by the roots of 
sedges and bulrushes. Above this the color and structure changed abruptly 
to gray, the peat became silty, and, instead of the sedge, the vegetable matter 
was practically all composed of the underground stems of the salt marsh 
grasses, now forming the covering of the marsh. 
The story is plain, and scarcely needs interpretation, but it is certain 
