62O STRUCTURE AND LIFE HISTORIES 



Silurian period to the present? No one knows. From a 

 study of the thickness of rock strata, and a knowledge of 

 the probable time required for the depositing of those 

 strata as sediment on the floor of the ancient oceans, and 

 their elevation and denudation to their present condition 

 by weathering and erosion, geologists have been able to 

 suggest relative measures of geologic time. Paleozoic 

 time is long, twice as long as Mesozoic time, and Meso- 

 zoic time must be at least twice as long as Cenozoic time. 

 The actual age of the earth is, however, a problem which 

 engages the attention of physicists as well as geologists. 

 Sixty years ago Lord Kelvin gave a mean estimate of 

 100,000,000 years. With this estimate the geologists, 

 Walcott and Geikie, have nearly concurred; but since the 

 discovery of radium it has been estimated that certain 

 carboniferous iron ores have an age of 140,000,000 years. 



Figures of such magnitude convey but little meaning to 

 our minds; they are too large for us to grasp their real 

 value. "Therefore," as Darwin has said, "a man should 

 examine for himself the great piles of superimposed strata, 

 and watch the rivulets bringing down mud, and the waves 

 wearing away the sea-cliffs, in order to comprehend some- 

 thing about the duration of past time, the monuments of 

 which we see all around us." 



630. The Essence of Science. A careful reading of 

 this book will have led the student to realize that the un- 

 solved problems of botany are more numerous and quite 

 as interesting as those we have solved. The essence of 

 science is the endeavor to ascertain by the best method 

 that which is most worth knowing. 



