What is the difference between radioactive dating and radiometric dating
Suppose there exist a number of fossil-bearing outcrops each composed of sedimentary layers that can be arranged in relative order, primarily based on superposition.
Suppose, too, that all the layers contain a good representation of the animal life existing at the time of deposition.
Such features as colour, ripple marks, mud cracks, raindrop imprints, and slump structures are directly observable in the field.
Properties derived from laboratory study include (1) size, shape, surface appearance, and degree of sorting of mineral grains, (2) specific mineral types present and their abundances, (3) elemental composition of the rock as a whole and of individual mineral components, (4) type and abundance of cementing agent, and (5) density, radioactivity, and electrical-magnetic-optical properties of the rock as a whole.
Correlation based on the physical features of the rock record also has been used with some success, but it is restricted to small areas that generally extend no more than several hundred kilometres.
The first step is determining whether similar beds in separated outcrops can actually be traced laterally until they are seen to be part of the same original layer.
On the practical side, an index fossil should be distinctive in appearance so as to prevent misidentification, and it should be cosmopolitan both as to geography and as to rock type.The more ways in which two rocks are physically alike, the more likely it is that the two formed at the same time.Only a partial listing of physical characteristics is necessary to indicate the breadth of approach in this area.The basis for assuming that like fossils indicate contemporary formation is faunal succession.However, as previously noted, times of volcanism and metamorphism, which are both critical parts of global processes, cannot be correlated by fossil content.
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Nevertheless, there is no greater testimony to the validity of fossil-based stratigraphic geology than the absolute dates made possible through radioactive measurements.