Radiometric dating means
For Uranium/Lead dating this means that some of the uranium that was initially present would be "leached" out of the rock.
That's because radiometric dating (with the exception of Carbon 14) is almost always performed on igneous rocks (i.e. Also because, when different substances are in a liquid state, something known as mixing almost always takes place: meaning that whenever a liquid (or molten) rock is erupted out of the earth, both the mother and daughter elements will be "mixed" together, thus making it virtually impossible to determine the time that an eruption took place.The second assumption is much more speculative since there is no way to verify whether or not some (or most) of the daughter element was already present when the rock solidified. However, in some cases, a few scientists are telling us that they have solved this problem.For example, with the uranium/lead method scientists have attempted to estimate what the original ratio (of uranium-238 to lead-206) was when the Earth formed.And since the only rocks which yield ages in excess of 100,000 years are of volcanic origin, this method of dating the earth is not based on science, but rather speculation and subjective reasoning.Unfortunately, the public is rarely informed of these facts. * "Allende" is the name given to the meteorite that was used to "date" the age of the earth. It is the site where the famous 1470 skull was found.