Dating methodology for the Vishnu Schist

The answer is quite simple - your calculations deal with the average rate taking into account all additive and subtractive processes and in terms of rock, not sediment,One lesser point is that sedimentary rock is compressed, so that it occupies less volume than the equivalent loose sediment.But more importantly there have been times where there was erosion - quite considerable erosion. The base of the Grand Canyon - including the Vishnu Schist represents the roots of mountains that were raised - and destroyed by erosion. The boundary between these rocks and the higher, later rocks is known as the "Great Unconformity"See the picture at the bottom of this pageThere are more - if less impressive - examples of erosion between strata further up the canyon.

That is where radiometric methods come in very handy. Methods using longer-lived radioactive substances do give an absolute age - although most methods only work on igneous rocks (there are other things to beware of, too - for instance periods of heating can reset the "clock").It is possible to make rough estimates based on depth and the early geologists did. It is possible to find evidence that erosion has occurred - and, at least sometimes, the eroded strata can be still found in other locations. For all I know it may still be used to fill in the gaps between more solid dates.