186 THE STOSY OP THE EARTH AND MAN. 



and it will be seen that eacli age constitutes a 

 cycle, similar in its leading features to the other 

 cycles, while each is distinguished by some important 

 fact in relation to the introduction of living beings. 

 In this table I have, with Mr. Hull,* for simplicity, 

 arranged the formations of each age under three 

 periods — an older, middle, and newer. Of these, 

 however, the last or newest is in each case so im- 

 portant and varied as to merit division into two, in 

 the manner which I have suggested in previous pub 

 lications for the Palaeozoic rocks of North America.f 

 Under each period I have endeavoured to give some 

 characteristic example from Europe and America, 

 except where, as in the case of the coal formation, 

 the same names are used on both continents. Such 

 a table as this, it must be observed, is only tentative, 

 and may admit of important modifications. The 

 Laurentian more especially may admit of division 

 into several ages; and a separate age may be found 

 to intervene between it and the Cambrian. The 

 reader will please observe that this table refers to 

 the changes on the continental plateaus ; and that 

 on both of these each age was introduced with shallow 

 water and usually coarse deposits, succeeded by 

 deeper water and finer beds, usually limestones, and 

 these by a mixed formation returning to the shallow 

 water and coarse deposits of the older period of the 

 age. This last kind of deposition culminates in the 

 great swamps of the coal formation. 



* " Quarterly Journal of Science," July, 1869. 

 t « Acadian Geology," p. 137. 



