THE PALEONTOLOGICAL RECORDS 297 



the surface of the earth. The latter was, at a remote time from 

 now, a mass of mohen-gaseous material which broke away from 

 the parent sun — that was probably between one and two thousands 

 of millions of years ago. When this gaseous-molten mass 

 solidified and became cool water from the primitive gaseous 

 envelope condensed upon it and formed the first seas — that we 

 take to be about 1,200, or more, millions of years ago. As soon 

 as the first stable, solid crust and seas appeared there must have 

 been erosion of land and consequent sedimentation. 



The approximate ages of the various systems are given in 

 millions of years. These results depend mainly on investigation 

 of the rate of radio-active change which is imperfectly recorded 

 in the compositions, and appearances presented by certain 

 minerals. It is probable that the ages of the systems cannot be 

 less than those given. 



The process of sedimentation has not been a continuous one. 

 Between some of the systems — as, for example, between the 

 Proterozoic and Paleozoic rocks, as we know them, there are 

 " unconformities," that is, there were other sedimentary rocks 

 there that have been eroded and destroyed. At irregular intervals 

 there have been geological '' revolutions " in the course of which 

 the seas have *' transgressed " on the land ; large parts of the 

 latter have been eroded ; mountain ranges have been formed — 

 and eroded. In general great earth- disturbances have occurred. 

 At present we live in the period of earth- quiescence that has 

 followed the last (Alpine) revolution. 



Throughout the entire period represented in the table life has 

 been possible — and has probably existed, first in the seas and then 

 on the lands. The table presents a general summary of the 

 occurrence of life, but this we shall consider presently. 



98(2. FossiLiZATiON. Fossils are artifacts that result from 

 the preservation, with alteration, of the hard or soft parts of the 

 bodies of plants and animals. As a rule, it is the hard parts such 

 as the shells of molluscs, the hard parts of Echinoderms, the scales 

 and endoskeletons of Fishes, etc., that become preserved as fossils. 

 But even leaves and the soft tissues of animals may exceptionally 

 become preserved. In the ordinary processes of fossilization the 

 dead body of the organism becomes covered by fine mud, or silt, 

 that retards decomposition and may not crush the body. The 

 shell, skeleton, etc., then, usually become mineralized, that is. 



