dubois@uwmacc.UUCP (Risky Rat) (07/31/85)
Ross H Arnett, Jr and Dale C Braungart, _An Introduction to Plant Biology_ (third edition), C V Mosby, Saint Louis, 1970, pp311,313. "Geologists have worked out a scale, the geological time scale, which names the various periods of time since the beginning of the earth. This scale is based primarily on the layers of stratified rock. Volcanic rock is not dated in this manner because it cannot be measured in time units. It is possible to date both types of rock by radioisotope studies. The original time scales were dated by measuring the thickness of each layer, and an estimate was then made of the thickness of known silt deposits for a few years. By careful computations, and [sic] estimate of the time it took for a given deposit to accumulate could be made. After long study, the various deposits were cooordinated and arranged into a time scale. Actually, the geologists used fossils to help date the rocks, and this practice resulted in a paradox because the paleontologist used the rocks to date his fossils! However, when it became possible to date organic matter by C-14 tests, the entire time scale was in need of only minor adjustments." Comment: it is indeed distressing that carbon-14 is now only accurate to at most 50,000 years, since in 1970 even Pre-Cambrian deposits could be dated by that method. Most unfortunate indeed! :-) ...One would think that by the third edition such an error would be detected... -- | Paul DuBois {allegra,ihnp4,seismo}!uwvax!uwmacc!dubois --+-- | "Why are you standing on one leg?" | "I'm trying to see if I'm a stork."