Pace Four 



EVOLUTION 



How Old Is the World? 



July, 1928 



By Allan Strong Broms 



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GEOLOGIC 

 DIVISIONS 



CR£tACE0U5 



COMANCHIAN 



PENKSYLVANIAN 



MISSIS5IPPIAN 



SILURIAN 



OROOVICIAN 



IJECENTLY published scientific measurements of the 

 **• age of the earth increase rather than decrease the 

 hundreds of millions of years since the formation of 

 the earliest known rocks. In the accompanying diagram 

 the consensus of conservative geological opinion is 

 shown. The oldest known rocks are given an age of 

 1,500 million years, but the actual figure may be as 

 great as four billion. 



The most trustworthy means of deter- 

 mining the ages of the rocks as the 

 uranium-lead method, already described 

 in the February issue of Evolution. We 

 know the rate at which the radio-active 

 mineral uranium breaks down into lead. 

 By measuring the relative amounts of 

 uranium and of lead in a given rock, 

 we can fix its age very closely by a 

 simple computation. The conservative 

 results (tabulated by Joseph Barrell) 

 are shown in the diagram. 



The figures of the time scale are given 

 in millions of years. Read it from the 

 bottom up, from the earliest geologic 

 eras upward to the present, that being 

 ttie order in which the various rock 

 strata have been laid down. 



The Geologic Divisions 

 Five great geologic eras are shown. 

 The Cenozoic (most recent) is relatively 

 so short that there is no room for show- 

 ing its subdivisions, which in order of 

 time are: Eocene, Oligocene, Miocene, 

 Pliocene, Pleistocene (Glacial Period I 

 and Recent periods. The Mesozoic and 

 Paleozoic eras are shown fully sub- 

 divided into periods, but the subdivisions 

 of the earlier Proterozoic and Archeozoic 

 eras are variously classified and are, 

 accordingly, not shown here. Between 

 these two earliest eras occurs a great 

 geologic gap, during which the entire 

 geologic record was wiped out by ero- 

 sion. We know little about it other 

 than its <;real. though unrertain, length. 



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indicates a cycle in geologic forces and accords very 

 well with Professor Joly's theorv of geologic revolutions 

 described in the April issue of Evolution, except that 

 his estimates of the time between revolutions appear too 

 short. But this does not argue against his theory at all. 

 He holds that the heat from radioactivity slowly accu- 

 mulates within the earth, eventually melts the substratum 

 and the thinner crustal rocks. By expansion the sub- 

 stratum forces the crust as 



MOUNTAIN 

 BUILDING 



HIMALAYAS 

 CASCADES 

 EUROPE 

 E ALPS 



LONG 

 EROSIONAl 

 INTERV- 



ALaOMAN 



LAUflENTIAN 

 N EUROPE 





a whole outward and yet 

 permits the mountainous 

 areas of the crust to sink 

 deeper into its own less 

 dense substance. As por- 

 tions of the earth crust are 

 melted thinner, a rapid 

 cooling, solidifying and 

 shrinking of the substra- 

 tum occurs. As the crust 

 settles upon the shrunkea 

 interior, it wrinkles into 

 great folds, valleys and mountain ranges. 

 Also the heavier (because cooled) sub- 

 stratum now forces the lighter crustal 

 rocks upward as great mountain ranges. 

 Of course, the heat accumulation begins 

 once more, slowly preparing for the next 

 revolutionary cycle. 



Periods of Mountain Building 

 Without going into details, the great recorded periods 

 of mountain building are indicated. Only the 

 Cenozoic mountains are still high and jagged, i. e., 

 young in the geologic sense. Even such recently built 

 ranges as the Appalachian show the signs of wear, their 

 tops being well worn down and rounded. Many of the 

 earliest ranges have been completely leveled bv the 

 age-long erosional forces. 



It will be noted that the great periods of mountain 

 building are spaced at fairly regular intervals. This 



The Evolution of Life 



Bearing most closely upon the prob- 

 lem of evolution is the record of life in 

 the rocks. In the first two eras (Archeo- 

 zoic and Proterozoic) the life forms 

 were so primitive and soft-bodied that 

 they left a very doubtful record, and 

 that record largely erased by long and 

 extreme changes. 



When the simple sea life of the early 

 Paleozoic developed shells protecting 

 them from attack, and perhaps from the 

 increasing saltness of the oceans, they 

 left a real record that would stand the 

 wear and tear of time. From their evo- 

 lutionary progression we can now classify and correlate 

 quite definitely the several geologic periods of this and 

 more recent eras. 



By the middle of the Paleozoic, fishes, the first back- 

 boned animals, appeared. Towards its end, the am- 

 phibia (frogs, salamanders and newts being modern rep- 

 resentatives) took the very important step from water 

 to land and developed lungs. 



The Mesozoic Era is dominated by the reptiles, which 

 culminated in the great land and sea lizards, the dino- 

 saurs, most amazing of our museum pieces. They were 

 not as large as our whales, but they were built with much 

 bulk and brawn, though with hardly any brains. 



Geological Time-table 



