THE STORY OF NINETEENTH-CENTURY SCIENCE 



formed on first cooling from a molten state ; but they 

 are now more generally regarded as once-stratified de- 

 posits metamorphosed by the action of heat. 



Whether " primitive " or metamorphic, however, these 

 Canadian rocks, and analogous ones beneath the fossil- 

 iferous strata of other countries, are the oldest portions 

 of the earth's crust of which geology has any present 

 knowledge. Mountains of this formation, as the Adi- 

 rondacks, and the Storm King range overlooking the 

 Hudson near West Point, are the patriarchs of their 

 kind, beside which Alleghanies and Sierra Nevadas are 

 recent upstarts, and Rockies, Alps, and Andes are mere 

 parvenus of yesterday. 



The Laurentian rocks were at first spoken of as repre- 

 senting "Azoic" time; but in 1846 Dawson found a 

 formation deep in their midst which was believed to be 

 the fossil relic of a very low form of life, and after that 

 it became customary to speak of the system as " Eozoic." 

 Still more recently the title of Dawson's supposed fossil 

 to rank as such has been questioned, and Dana's sug- 

 gestion that the early rocks be termed merely Archaean 

 has met with general favor. Murchison and Sedgwick's 

 Silurian, Devonian, and Carboniferous groups (the ages 

 of invertebrates, of fishes, and of coal plants respective- 

 ly) are together spoken of as representing Paleozoic time. 

 William Smith's system of strata, next above these, once 

 called " secondary," represents Mesozoic time, or the age 

 of reptiles. Still higher, or more recent, are Cuvier 

 and Brongniart's Tertiary rocks, representing the age of 

 mammals. Lastly, the most recent formations, dating 

 back, however, to a period far enough from recent in 

 any but a geological sense, are classed as Quaternary, 

 representing the age of man. 



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