Some of these rocks are sedimentary, and include minerals which are themselves as old as 4.1 to 4.2 billion years.

Rocks of this age are relatively rare, however rocks that are at least 3.5 billion years in age have been found on North America, Greenland, Australia, Africa, and Asia.

The higher the uranium-to-lead ratio of a rock, the more the Pb-206/Pb-204 and Pb-207/Pb-204 values will change with time.

If the source of the solar system was also uniformly distributed with respect to uranium isotope ratios, then the data points will always fall on a single line.

The most direct means for calculating the Earth's age is a Pb/Pb isochron age, derived from samples of the Earth and meteorites.

This involves measurement of three isotopes of lead (Pb-206, Pb-207, and either Pb-208 or Pb-204).

he generally accepted age for the Earth and the rest of the solar system is about 4.55 billion years (plus or minus about 1%).

This value is derived from several different lines of evidence.

(I believe this argument was originally put forth by Mormon young-Earther Melvin Cook, in a letter to the editor which was published in .) But helium can and does escape from the atmosphere, at rates calculated to be nearly identical to rates of production.

In order to obtain a young age from their calculations, young-Earthers handwave away mechanisms by which helium can escape.

The actual underlying assumption is that, if those requirements have not been met, there is no reason for the data points to fall on a line.