8 THE EVOLUTION OF MAN. 



crust of the earth. The result of this unceasing work of 

 the water, which in the form of rain and hail, of snow and 

 ice, of rushing torrent and surging wave crumbles and dis- 

 solves the rocks, is the formation of ooze. As Huxley says, 

 in his excellent "Lectures on the Causes of the Phenomena of 

 Organic Nature," the most important fact in the past history 

 of our earth is ooze, and the question as to the history of the 

 past ages of the world resolves itself into a question as to 

 the formation of ooze. All the stratified rocks of our moun- 

 tainous formations were originally deposited as ooze at the 

 bottom of the waters, and only afterwards hardened into 

 solid stone. 



As has already been said, it is possible, by bringing 

 together and comparing the various rock-strata from many 

 places on the surface of the earth, to obtain an approximate 

 conception of the relative ages of these various strata. 

 Geologists have long agreed that there is an entirely definite 

 historical sequence of the various formations. The various 

 groups of strata which lie one over another correspond to 

 successive periods in the earth's organic history, during 

 which they were deposited in the shape of mud at the 

 bottom of the sea. Gradually this mud was hardened into 

 solid rock. The latter, by alternate upheaval and depres- 

 sion of the surface of the earth, was lifted above the water, 

 and assumed the form of mountains. Four or five main 

 periods in the earth's organic history, answering to the 

 larger and smaller groups of these sedimentary rock-strata, 

 are usually distinguished. These main periods are sub- 

 divided into numerous subordinate or lesser periods. From 

 twelve to fifteen of the latter are usually assumed. (Cf. 

 Tables XII. and XIII., pp. 11, 12.) The relative thick- 



