The geological timescale

The Geological Time-Scale is hierarchical, consisting of (from smallest to largest units) ages, epochs, periods, eras and eons. Each era, lasting many tens or hundreds of millions of years, is characterized by completely different conditions and unique ecosystems. 

It is followed the geological time scale as determined by the International Commission on Stratigraphy (ICS). The ICS has not finished its job and gaps remain, particularly in the Early Paleozoic. Where gaps occur, it is generally followed the Russian system for the Cambrian and the British system for the Silurian. Epochs are subdivided further into ages not listed here. The periods from Cretaceons and older are subdivided into epochs and ages not shown here. 

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The geologic timescale for the Earth is based mostly on stratigraphy (the depositional succession of layers from bottom to top). Most strata are sedimentary rocks. The relative ages of the sedimentary layers are determined by the sequence of deposition and by the fossils that they contain. It is not possible to use radioactive isotopes to measure the time that a sediment was deposited, because deposition does not reset radiometric clocks. However, the absolute ages of sediment layers and the fossils they contain have been determined by measuring radiometric ages of volcanic ash layers in the sequence or lavas that crosscut sedimentary strata. 

Heimann and Maier-Reimer, 1996 Morimoto et al., 2000 Tans et al., 1990, 1993 Battle et al., 2000 Still et al., 2003). As is the case in the geological timescale studies, carbon isotopes offer a simple and powerful approach that is based, to a great extent, on the existence of the minute but easily detected biological discrimination against in photosynthesis. This isotopic signal is useful in two ways ... 

Q-Quartz, which has a trigonal crystal structure, undergoes a rapid, reversible transition to hexagonal /J-quartz at 573 °C and then slowly changes to hexagonal /3-tridymite at about 870 °C tridymite in turn goes over slowly to cubic /3-cristobalite at 1470 °C, and this melts at 1713 °C. The reversion of cristobalite and tridymite to quartz is slow, so that these forms can exist at room temperature (as a-modifications). In addition, dense modifications with six-coordinate Si are found in shocked rocks associated with meteorite impact craters coesite forms only above 450 °C and 3.8 GPa, and stishovite requires over 1200 °C and 13 GPa. Survival of those metastable polymorphs on the geological timescale is evidence of an extremely slow recrystallization rate. 

The U-Pb date of zircon from the Gorecki Felsite (5(X) 8 Ma) confirmed that the age of the Patuxent Formation in the Schmidt Hills is in fact. Middle to Late Cambrian according to the geologic timescale of the lUGS (2002). 

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