66 THE DOCTRINE OF DESCENT. 



lower one chalky (mountain limestone), the middle on-.^ 

 conglomerated or arenaceous (millstone grit), and the 

 upper one carboniferous (coal measures); hence a marine, 

 a littoral and a marsh or fresh-water formation. It is 

 easy to imagine the cause of this phenomenon ; it de- 

 pends on the secular elevation of the primaeval sea 

 bottom, on which was deposited first the marine moun- 

 tain limestone ; secondly, as it rose to the surface, the 

 shingle and coarse sand of the shore; and finally, on 

 persistent elevation, the products of marshes, lagunes, 

 and estuaries. If it now happened that some portions 

 of the infant continent covered with the latter (that is 

 to say, with the productive carboniferous strata), were 

 seized with an opposite movement, and therefore sank, 

 there would be deposited on the surface now again 

 gradually becoming the bed of the sea, precisely similar 

 forms, only in inverse order to that which occurred 

 .during the period of elevation. 



And, in fact, this phenomenon is exhibited by those 

 portions of the earth's surface which shortly after the 

 formation of the coal measures again sank below the 

 sea. In Germany and England the productive coal 

 measures are followed by a sandstone and conglomerate, 

 therefore a littoral formation, exactly like the quartzose 

 sandstone and millstone grit which underlies them ; and 

 above this a limestone, dolomite and gypsum formation, 

 corresponding to the mountain limestone, the lowest 

 member of the carboniferous system. On account of 

 the division which is displayed in profound palaeon- 

 tological and petrographical diversities, the formation 

 thus developed and composed is designated as the 

 Dyas. The separate phases of this cycle of occur- 



