Geological Aspects of Evolution 95 
years ago. The next question which logically follows is, upon 
what are these estimates based? There are several methods but 
only two will be briefly touched upon here. One is based upon 
the known thickness of sedimentary rocks and the rate at which 
these are being deposited today. Unfortunately, the rate of 
sedimentation varies widely at different places. But even if we 
knew the average rate during the deposition of the known 50 
miles of sedimentary rocks deposited since the beginning of 
Cambrian time, the figure so obtained would be a minimum. For, 
the rocks are separated at many places by unconformities or 
erosion intervals in which deposition of the products of erosion 
took place beyond the limits of the present continents and also 
by under-water breaks or diastems. Another method is known 
which proceeds at a constant rate without interference. Radio- 
active elements, such as uranium, disintegrate and give off par- 
ticles at a rate which has been determined and yield as an end- 
product, lead with an atomic weight different from ordinary lead. 
Eight stages have been established in the radium disintegration 
Series, in which elements of lower atomic weights are formed at 
a measured rate. Thus, uranium with an atomic weight of 238 
is progressively changed by the loss of positively charged helium 
atoms each having an atomic weight of 4 until there is formed 
a stable product, uranium lead with an atomic weight of 200. 
Knowing the uranium-lead ratio and the rate at which atomic 
disintegration proceeds, it is possible to determine the age of the 
mineral and the geologic formation in which it occurs. By this 
method the oldest rock, which is of Archeozoic age, is 1,850,- 
000,000 years old, and those of the Upper Cambrian which is 
the first period of abundant invertebrates, are 450,000,000 years 
old. Allowing time for the deposition of the Middle and Lower 
Cambrian, the beginning of the Paleozoic is estimated at 500,- 
,000 years ago. 
Some of the great erosion intervals occurred at the close of 
the eras and were caused by mountain-making disturbances, which 
folded and tilted the originally flat-lying sediments. These move- 
ments changed the distribution of land and sea and the life of the 
Sea and the land was affected by each new disturbance. The close 
