86 THE PRIMARY PERIODS. 



modern geologists arrange the earth's crust into the follow- 

 ing formations, or, referring to their fossils, into the follow- 

 ing life-periods and rock-systems : 



Life-Periods. Rock-Systems. 



( Quaternary or Recent. 

 CAINOZOIC= RECENT LIFE. \ 



( Cretaceous or Chalk. 

 MESOZOIC=MIDDLE LIFE. \ Oolitic or Jurassic. 



V Triassic or Upper New Red. 



("Permian or Lower New Red. 



I Carboniferous or Coal System. 

 ?ALJ30ZOIC=ANCIENT LIFE. < OM Red Sandstoiie and Devonian. 



[Silurian. 

 < Cambrian. 

 EOZOIC=DAWN LIFE. \ Laurentian. 



This arrangement is, of course, temporary or provisional, 

 and may be altered as geologists "become more intimately 

 acquainted with rocks of other lands ; but in the mean time 

 it expresses our knowledge of the succession that prevails 

 among the stratified formations, and may be received as the 

 great chronological stages of the world's history. The tech- 

 nical terms are founded partly on mineral, partly on geo- 

 graphical, and partly on fossil distinctions, and may with 

 a little exertion become intelligible to the least scientific. 

 Thus " Cretaceous" and " Old Eed Sandstone" refer to the 

 most prevalent rocks in these systems ; "Laurentian" and 

 " Cambrian " to districts where the systems are largely 

 or typically displayed ; and " Cainozoic" and " Paleozoic" 

 to the comparative recentness and antiquity of the fossil 

 remains. A uniform system of nomenclature might have 

 been preferable, but human knowledge progresses by slow 

 degrees, and its terms and technicalities must be viewed 

 as mere provisional expedients towards this advancement. 

 Under the existing nomenclature geology has made bold 

 and rapid progress, and any attempt to revolutionise, unless 

 through the gradual increase of wider knowledge, would 



