Page eight 



EVOLUTION 



February, 1931 



How Long Ago? 



By EDSON S. BASTIN 



/"' EOLOGY has made a notable contribution to an intelligent of such sediments. In other places they may be accumulating 



^-' philosophy of life by giving us at least a rough idea of faster or slower, and some sorts of sediments form much more 



the amount of time involved in the great pageant of life-de- slowly than others, but by many observations scattered over 



velopment. In geologic, as in human history we need to see the earth he can arrive at some rough average of the time 



events in their proper time perspective to understand fully necessary to accumulate a foot of these sediments. On the 



their meaning. Having placed the varied forms of the past in average it appears to take something like 900 years for a 



their proper positions in the great procession of evolving life, foot of sediment to be laid down. 



how shall we know whether the procession moved rapidly or Now if we multiply this period of time by the total nimiber 



slowly? The question is so difficult that it may never be com- of feet of sediments that have accumulated in the past to form 



pletely answered, but enough has been learned to show that our great series of sedimentary rocks — 70 miles in thickness in 

 profoundly long periods of time are involved. 



How does the geologist solve a prob- 

 lem of this sort? There is no mystery 

 in his methods. One of his simple guid- 

 ing principles is that processes going on 



in the earth's surface today went on also 

 in the past — the present is the key to the 

 past. In the shallow waters of the shores 

 of Lake Michigan he sees the sand 

 marked by a series of little ridges and if 

 he watches he can see that these are 

 formed by the drag of the waves on the 

 lake bottom. He calls them ripple- 

 marks. 



His next excursion is to a quarry where 

 hard rock, sandstone, is being quarried. 

 The quarryman with his hammer splits 

 a piece off a solid, flat ledge and there 

 on its surface are ridges or ripple marks 

 precisely like those he saw forming on 

 the lake bottom. The natural and rea- 

 sonable conclusion is that what is now 

 sandstone was once sand shifted by the 

 waves in the shallows of a lake or the 



100 MWhon yeacs 



Graph illustracing geologic time and the 

 evolution of earth's history. From David 

 White, U. S. Geological Survey. 



we arrive at a figure of around three hundred million years 

 as the time necessary for them to accu- 

 mulate. Crude as is this method and 

 fraught with many uncertainties, it yet 

 gives us some rough notion of the im- 

 mense periods of time involved in the his- 

 tory of the earth. 



Another method of calculating geo- 

 logic time is based on the amount of 

 common salt in the oceans. Water fall- 

 ing as rain on the earth's surface dis- 

 solves salts of various kinds as it flows 

 off over the surface or underground, 

 eventually to join the brimming river and 

 travel to the sea. Some of the salts are 

 deposited when they reach the ocean, but 

 most of the common salt remains dis- 

 solved in the ocean waters. It is possi- 

 ble to calculate roughly, from the known 

 size of the oceans, how much common 

 salt they contain. It is also possible to 

 calculate roughly how much common salt 

 is being carried to the ocean in a year by 

 the rivers of the world. Dividing the 

 first figure by the second gives us a figure 



On another exiursion to a new locality, the geologist finds of around 100 million years for the age of the ocean. Obvi- 

 sandstone beds showing the same sort of markings, but this ously this method of calculation has many grave uncertainties 

 time the ripple marked surfaces are standing vertical instead and is valuable only in giving a rough conception of the general 



of lying flat. From this he concludes, not only that lake or 

 ocean waters once stood where now is a sandstone quarry, but 

 also that since then the sandstones have been tilted by earth 

 movements from their original flat-lying position until now 

 they stand vertical. 



Similarly in Italy the geologist sees rocks of peculiar kinds 

 forming today from cooling lavas of Vesuvius. When later, 

 on our New England coast, he finds rocks of almost identical 

 sorts he concludes that once volcanoes were active there altho 



order of magnitude of the periods of time involved in geologic 

 history. 



The newest method of measuring the age of the earth is by 

 far the most ingenious and probably the most reliable. There 

 are substances that are continually disintegrating by giving off 

 what are commonly called rays or emanations. The radium or 

 thorium salts used for the luminous figures on the dials of 

 clocks and watches are substances of this sort. While many 

 of the substances that thus disintegrate spontaneously are pro- 



they have long since ceased to belch forth molten lavas and duced only in the laboratory, some are minerals occurring na- 



their once mountainous forms have been worn down to low- 

 lying plains. 



It is by methods equally logical, if not quite as simple, that 

 he learns something of the age of the earth. At the mouths 

 of rivers and along shores he sees rocks in process of forma- 

 tion as muds and clays and sands. By careful watching he 

 can learn how long it takes to accumulate an inch or a foot 



turally in the rocks. Now the significant point is that the 

 rate at which such minerals disintegrate is nearly constant and 

 can be measured in the chemical laboratory in terms of our 

 ordinary time units of hours and years. Furthermore when 

 such minerals disintegrate, certain of the products of disintegra- 

 tion remain in their entirety associated with the original min- 

 eral. If then we determine by analyzing their proportions in 



