724 TEXTBOOK OF BOTANY 



to be formed. Unfortunately, after the sediments had become rocks, 

 the rocks were subjected to such enormous pressure, to such high 

 temperatures, to the forces of crystalhzation, and were so altered that 

 little remains to mark the fossils that undoubtedly occurred in these 

 rocks when the)' were young. 



In such metamorphosed rocks we can infer the existence of plants 

 from certain structural features of the rocks or from certain mineral 

 deposits that occur among them. For example, in the regions around 

 the west end of Lake Superior are great deposits of iron in pre-Cambrian 

 rocks. In view of what we know about the deposition of comparable 

 deposits in recent times we have a right to infer that iron bacteria were 

 living in the water where these deposits accumulated. Nearby are de- 

 posits of graphite, a compact form of carbon, which is the end product 

 when crushing pressures, heat, and recrystallization have acted upon 

 coal which in turn is a derivative of peat and other plant debris. Small 

 amounts of carbon in metamorphic rocks might possibly have a dif- 

 ferent origin, but large deposits represent much larger masses of plant 

 remains. 



If one examines the natural processes of the present, he can roughly 

 estimate how many of the plants of today are falling into situations 

 where they, or their imprints, may be preserved. He will then more 

 fully realize how few are the land plants that will leave in the earth a 

 record that might be found a hundred years from now. He will also 

 understand more clearly whv the fossil plant record is so meager and 

 so scattered. We have no reason to assume that on the continent as a 

 whole the conditions favoring the preservation of plants were very 

 different from what they are now, except for the interference of man. 



Of course there were long periods of time when shallow seas and 

 lakes were more extensive. For example, during the Cretaceous period 

 there was an open waterway between the Gulf of Mexico and Alaska 

 where now are the high plains. As the land rose toward the close of 

 the Cretaceous period, this waterway became a series of fresh-water 

 lakes and swamps. At the present time the northern states and Canada 

 are also marked by hundreds of thousands of lakes in depressions left 

 by the vast continental glaciers, the last of which disappeared only 

 about 10,000 years ago. 



Extensive forested swamps existed during the Carboniferous time 

 (Fig. 360), and in them the trees and other plants of those periods ac- 

 cumulated. From the consolidated and altered plant remains the eastern 



