THE MESOZO1C AGES. 193 



the production of nearly 1,000 feet in thickness of 

 chalk, a rock which, being composed almost entirely 

 of microscopic shells, is, as we shall see in the sequel, 

 necessarily of extremely slow growth. If we regard 

 the Cretaceous group as one of our great ages or 

 cycles, it seems to be incomplete. The sandstones 

 and clays known as the Greensand and Gault con- 

 stitute its lower or shallow-water member. The chalk 

 is its middle or deep-sea member, but the upper 

 shallow-water member is missing, or only very locally 

 and imperfectly developed. And the oldest of the 

 succeeding Tertiary deposits, which indicate much 

 less continuous marine conditions, rest on the chalk, 

 as if the great and deep sea of the Cretaceous age 

 had been suddenly upheaved into land. This abrupt 

 termination of the last cycle of the Mesozoic is obvi- 

 ously the reason of the otherwise inexplicable fact 

 that the prevalent life of the period ceases at the top 

 of the chalk, and is exchanged immediately and with- 

 out any transition for the very different fauna of the 

 Tertiary. This further accords with the fact that the 

 Cretaceous subsidence ended in another great crum- 

 pling of the crust, like that which distinguished the 

 Permian. By this the Mesozoic time was terminated 

 and the Cainozoic inaugurated; while the Rocky 

 Mountains, the Andes, the Alps, and the Himalayas, 

 rose to importance as great mountain ranges, and the 

 continents were again braced up to retain a condition 

 of comparative equilibrium during that later period of 

 the earth's chronology to which we ourselves belong. 



