68 EVOLUTION OF THE EARTH 



Relative duration of the eras. The preceding table gives 

 the generalized thicknesses of the sedimentary rocks of North 

 America in terms of the detritals (muds and sands) and the 

 solution materials (limestones and dolomites). To attain 

 more clearly to the relative duration of each of the eras it is 

 desirable to restate the probable time taken by the far more 

 slowly accumulating solution materials in terms of those of 

 the detritals. Geologists differ greatly as to the time required 

 to deposit any kind of sediment, but most of them will agree 

 that limestones take from four to ten times longer than the 

 coarser materials. On the other hand, the rate of deposition 

 is exceedingly variable from place to place and from time to 

 time. Further, the limestones of pre-Cambrian time are on 

 the average far less pure than those of the later eras. Ac- 

 cordingly, on the basis of per annum rate of sedimentary accu- 

 mulation there are as yet no reliable estimates, and as we are 

 just beginning to appreciate that the geologic record is very 

 incomplete, any percentages given at present must be taken as 

 only suggestive of geologic time. A far more reliable stan- 

 dard is that of the rate of disintegration of radioactive 

 minerals, though here the estimates of geologic time, of not 

 less than i,6oo million years, appear to the writer to be 

 excessive by 50 per cent. 



Now let us compare the above results with the ratios ob- 

 tained by other geologists, but before doing so it should be 

 said that their statements relate almost always to post- 

 Proterozoic time. If, however, we follow Dana's ratios, it 

 is clear that he held in his famous text-book that about one- 

 half of geologic time lies back of the Cambrian. It is on this 

 basis that the following table has been prepared, and the 

 figures in parentheses restate the ratios in percentages of all 

 geologic time. 



