MYSTERIOUS FORMS OF LIFE. 67 



conclusion. Geologists have considered these facts, and have 

 settled on the principle that the long history of sedimenta- 

 tion has been divided into aeons corresponding to successive 

 conditions of the world. Names have been assigned to these 

 aeons. Thus, the first series of sediments formed the strata 

 which lie deepest of all. They are called Eozvic, and the / 

 aeon during which they were accumulating is the Eozoic jEon. f 

 We will not pause here to inquire what these sediments rested 

 on in other words, what kind of rocks formed the bed of the 

 sea, at the beginning of that ^on. The ocean must have had 

 some solid bottom ; but of course, it was a bottom formed when 

 there was no ocean; for otherwise, the Eozoic strata would 

 not be the bottom strata. 



The Eozoic GREAT SYSTEM of strata is at least fifty thou- 

 sand feet thick. In the next aeon the changed conditions 

 gave origin to changed strata. They constitute a Great Sys- 

 tem known as the PALAEOZOIC; and the time during which 

 this system of strata was accumulated, is the PALAEOZOIC 

 JON. Next after this, came the MESOZOIC J^ON, during 

 which the MESOZOIC GREAT SYSTEM of strata was accumu- 

 lated. Lastly, followed the C^N'-O-ZO-IC JEoN, which contin- 

 ues to the present. The strata formed constitute the CJENO- 

 zoic GREAT SYSTEM. Now, before we take another walk, 

 these names must be well learned. 



XII. MYSTERIOUS KORMS OK 



FOSSILS. 



EVERY one has noticed the curious forms found in the 

 Drift, which so much resemble shells and corals, and buttons 

 or beads. Often they lie loose in the soil ; and often we see 

 them imbedded in fragments of limestones and sandstones 

 which are sometimes bowlders transported from a distance, and 

 sometimes fragments derived from a neighboring ledge or out- 

 crop of stratified rocks. In the cliffs at Panama are occasional 

 traces of shells, both bivalve and univalve. The latter 



