bcKEL.] MICHIGAN. 195 
leport, but deposits of the same character are known to occur in neighboring States, 
s well as in the adjacent portions of Canada, but their entire distribution and their 
trecise relation to climatic and geological conditions, etc., have not been determined. 
n extent the marl beds vary from a few acres up to several hundred acres. Some 
If the Portland-cement companies, it is stated, have marl beds from 500 to 1,000 
eres in area, with an average depth of 20 feet or more. Im most instances these 
figures probably refer to two or more separate but perhaps closely adjacent beds. It 
t safe to say, however, that single beds from 100 to 300 acres in area and with an 
verage depth of 20 feet or more are not rare. 
In depth the marl beds vary from a few inches up to over 35 feet, as has been 
lemonstated by the writer by actual borings. Other observers report depths up to 
|0 and even in excess of 70 feet, which are no doubt reliable measures. 
The marl beds occur principally in the basins of existing lakes, but frequently 
jxtend beyond the present water margins and underlie the bordering swamp. They 
Ire present also in many instances beneath beds of peat or muck, from a few inches 
h several feet thick, on which tamarack and other trees grow. The presence of 
pari beds about the borders of existing lakes and at an elevation in some cases of 
K) or 15 feet above their surfaces shows that the lakes have been lowered, usually by 
he cutting down of their outlets, since the marl began to form. In some examples 
[eat occurs beneath extensive marl beds, and in a few cases two or three alternations 
if layers of peat and marl have been discovered. Usually, however, there is but 
ne bed of marl present, which rests on a sandy or clayey bottom. It is evident in 
111 instances that the marl was deposited in a lake, and that the swamps, or in some 
iistances the now well-drained tracts where it is found, were formerly flooded. 
