26 ANIMALS OF THE PAST 
durability to the dead fishes that rotted in their 
midst while yet they were only soft mud. 
For just as a plaster cast boiled in oil becomes 
thereby denser and more durable, so the oily 
and other matter coming from decomposing 
fish operated on the surrounding sand or mud 
so as to make it more compact.” 
It may not be easy to explain how it came 
to pass that fishes dwelling in salt water, as 
these undoubtedly did, were thus deposited in 
great numbers, but we may now and then see 
how deposits of fresh-water fishes may have 
been formed. When rivers flowing through a 
stretch of level country are swollen during the 
spring floods, they overflow their banks, often 
carrying along large numbers of fishes. As the 
water subsides these may be caught in shallow 
pools that soon dry up, leaving the fishes to 
perish, and every year the Illinois game asso- 
ciation rescues from the “ back waters” quan- 
tities of bass that would otherwise be lost. 
Mr. F. S. Webster has recorded an instance 
that came under his observation in Texas, 
where thousands of gar pikes, trapped in a lake 
formed by an overflow of the Rio Grande, had 
