72 



mav place the time anywhere from a few hundred thousand 

 to a million years or more. Eventually, however, the period 

 came to an end, and the crinoids, corals, and other evidences 

 of pure water disappeared. The limestone areas were 

 invaded by mud-bearing currents, which apparently caused 

 the local extinction of the reef-building corals. Thus we find, 

 at the opening of the Hamilton period, that in place of the 

 former wealth and beauty of the coral reef, organic life 

 presents a more sombre tone. 



The cause of this change in sedimentation was the shoaling 

 of the water, so that extensive areas, formerly covered by 

 the sea, became dry land, while other areas were converted 

 into great mud-flats, laid bare at low tide. With the 

 shoaling of the water, the influx of abundant fresh water 

 from the land was probably combined, so that these shal- 

 lower portions of the sea became fresh or brackish, rather 

 than salt. We have here an approximation to the conditions 

 which later gave rise to the extensive coal-swamps, and the 

 black shales may be regarded as partial attempts at coal- 

 making. As Professor William B. Rogers once said: "Nature 

 tried her hand at coal-making during these epochs." 



If we wish to gain a conception of these regions as they 

 appeared after the beginning of the Middle Devonian period, 

 we need only look at the extensive mud flats, which are laid 

 bare at low tide on our shores, and notice the black carbon- 

 aceous mud, in which mussels and periwinkles lie buried by 

 the thousands, waiting for the returning tide to restore 

 them to activity. Some such conditions prevailed at the 

 opening of the Mesodevonian period over all this region. 

 Coarse sediment was absent, suggesting feeble currents in 

 the shallow waters. Only fine silt and mud was spread out 

 by the tidal currents, and the vegetation on these mud flats 

 was slowly buried, and underwent a partial decay. Occa- 

 sionally somewhat more gritty sediments were carried in, 

 and at such times the pelecypods {Lunulicardium /rai^Ve, 

 Actinopteria inuricnta, etc.) seem to have flourished m 



