66 THE DOCTRINE OF DESCENT. 
lower one chalky (mountain limestone), the middle one 
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 primzval 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 palzon- 
tological and petrographical diversities, the formation 
thus developed and composed is designated as the 
Dyas. The separate phases of this cycle of occur- 
