620 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. 
