338 



HISTORICAL GEOLOGY 



Nevertheless calculations, based chiefly on the thickness of 

 sediments deposited, give a rough approximation to the truth, 

 sufficient to show that geologic history is exceedingly long. 

 It has been estimated that from 2,000,000 to 3,000,000 years 

 would be necessary for the deposition of the sand, mud, and 

 ooze which formed the thick Cambrian strata. Similar es- 

 timates made for later periods indicate that the majority of 

 them were of some such duration. Their combined length 

 must then have been many millions of years, a lapse of time 

 almost too vast for comprehension. 



QUESTIONS 



1. How can the extent of the sea at a particular time in geo- 

 logic history be ascertained ? 



2. Why should limestone be deposited close to the shore of a 

 low, densely forested land, but not near a rugged or less verdant 

 coast ? 



3. Why should the Cambrian system be thicker on the average 

 where it is made up of sandstone and conglomerate than where it 

 consists largely of limestone? 



4. Over which kind of a surface would a rising sea spread most 

 rapidly, a peneplain or a mountainous plateau ? Why ? 



5. At several points in the interior of the United States the 

 basal layers of the Cambrian sandstone contain great angular 

 bowlders of quartzite, granite, and other rocks. What do these 

 indicate about the Cambrian shore line at those particular places ? 



FIG. 337. 



FIG. 338. 



6. Compare the diagrams, Figures 337 and 338. In which do you 

 find evidence of the existence of an island in the Cambrian period ? 

 Can you suggest how the other has come to resemble it in general 

 structure ? 



