iii8 PRINCIPLES OF STRATIGRAPHY 



Columnar sections are designed to show the superposition and 

 relative thickness of the strata of the region which they represent, 

 provided they are drawn to scale. They serve their main purpose 

 in giving a quick and comprehensive view of the stratigraphy of a 

 region and in making comparison with other regions possible. If a 

 uniform set of scales, each a multiple of the others, could be adopted, 

 ready comparisons of published sections for dififerent regions would 

 be possible, and would greatly facilitate the work of correlation. 



Ideal sections are attempts to restore the conditions as they were 

 before deformation or erosion has taken place. The term is also 

 sometimes used for generalized cross-sections, but this is better 

 avoided. In so far as structure is eliminated, the columnar section 

 is an ideal section, but sections to which the term is best applicable 

 should show a wider relationship than is possible in a columnar 

 section. Fig. 152, page 739 and Figs. 157 and 158, page 743, are 

 examples of ideal cross-sections. 



The Length of Geological Time. 



Various estimates of the actual length of geologic time have been 

 attempted, the basis of most of such estimates being the rate of 

 deposition, ascertainable in modern river systems, or the rate of 

 erosion of river canyons, such as the Niagara, the Yellowstone, 

 Colorado, etc., and the rate of retreat of the Falls of St. Anthony. 

 (See Willii^ms-19.) If it can be ascertained that the beginnings 

 of erosion have a definite relation to some other event which itself is 

 of definite value in geochronology, a basis for a rational estimate of 

 the actual time duration is furnished. Such a relationship seems to 

 have existed between the beginnings of the Falls of Niagara and of 

 St. Anthony, and the end of the Pleistocenic glacial period. So 

 many questionable factors, however, enter into the problem, that 

 it is scarcely worth while, with our present incomplete knowledge, 

 to attempt much more than the most general estimate. Thus 

 Cambric, Ordovicic and Siluric time has been estimated at 10,000,- 

 000 years ; Devonic time at 2,000,000 years ; Mississippic to Permic 

 time at 5,000,000 years, making a total of 17,000,000 years for the 

 Palaeozoic. Mesozoic time has been estimated at 7,000,000 years, 

 Csenozoic at 3,000,000 years and Psychozoic at 50,000, marking a 

 total of 27,050,000 years since the beginning of Cambric time. This 

 estimate is conservative, others having made a much larger one. Thus 

 Dana's estimate of the age the earth was at least 48.000,000 years. 

 Geikie's estimate ranges from 100,000,000 to 680,000,000 years, 



