MICHIGAN ACADEMY OF SCIENCE. 
Ill 
shore, where there are strong currents and high waves constantly stirring 
up the sandy bottom. The same objection would prevent the assumption 
that these trees were sprouts from drifting trunks or branches. It seems 
apparent, then, that at the time these trees were seedlings, they were growing 
on soil, it may be on isolated bars or on the mainland, above the low water 
level, and that subsidence to the extent of some feet has gone on during their 
life-time, the number of years and the amount of subsidence could be found 
out by examination of the trees themselves. 
If it is assumed that plants have grown n the past in similar relations to 
their environment, as at the present time, which may safely be done, modern 
peat deposits at once become the key to the conditions under which all 
deposits of vegetable origin were formed in past geological times. Begin- 
ning with the Present, the whole series of peats, lignites, coals- and even 
graphites may give not only records of the plant types which have existed, 
but also of the conditions under which they grew, and thus supply many 
of the details of climate, precipitation, water level and of other factors con- 
trolling organic life, which are now unknown, because the records have 
been imperfectly interpreted. For example: Peat deposits, associated with 
bars and beaches of the glacial and post-glacial lakes, which existed in the 
basin of the Great Lakes, if buried by materials which are certainly con- 
temporaneous with the lakes, may give conclusive evidence of the depth of 
water over the surface on which the peat was formed, and, in this way, of 
the water level of the lake for that period. 
From principles which have been stated already, it is apparent that peat 
which has been formed on a wet land-surface, or in very shallow water, 
differs materially in constituent plants, and hence in structure, and other 
essentials, from that built up in deeper water of ponds and lagoons; that 
which has been formed by wave-worn drift material, brought together by 
wind and wave-formed currents, in eddies, behind bars and in similar places, 
where the current was slackened, is also easily distinguished from other types, 
and, where found, is obviously a record of importance to one seeking infor- 
mation relating to the shore deposits of these ancient lakes, and to water 
levels while they were being formed, provided that he can read them. 
Only recently, a specimen of peat was referred to the writer for examination 
from a peat bed connected with the shore deposits of Lake Chicago, and 
which had been taken by various observers to indicate a low-water stage 
in the lake. Examination showed, however, that it was made up of hetero- 
geneous drift material which probably accumulated in water of considerable 
depth, and which certainly did not form above water-level, or in a marsh, 
as a turf stratum. 
Coal and lignite often show remarkably diverse vegetable structure even 
in beds of the same horizon, or in different layers of the same bed; but, 
instead of considering this an anomaly, it is probable that, in the light which 
the study of the peat should give, it is to be expected under some conditions 
which favored the development of the vegetation from the remains of which, 
both coal, and lignite have been developed. 
In like manner, it is evident from the intimate association of peat formed 
from fresh and salt water vegetation, mentioned above, that it is not neces- 
sary to assume, as sometimes has been done, that coal which has marine 
fossils above and below, or even interbedded with it. was formed from marine 
algae or from plants growing on salt or brackish marshes. Marine fossils 
and the usually greater amount of ash constituents in the coal which occurs 
with them, when found at the top of coal strata, may simply mark the time 
