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The numeric values on the International Chronostratigrahpic Chart are represented by the unit Ma (megaannum, for 'million years'). For example, 201.4 ± 0.2 Ma, the lower boundary of the Jurassic Period, is defined as 201,400,000 years old with an uncertainty of 200,000 years.
The chart maintains a Victorian design, and compresses 6,000 years of history into a 23-foot wallchart. It is thoroughly illustrated with tables, and references. On the overall format of the chronology, the author remarks:
The ice age was probably not as long-lasting as once thought; study of oxygen isotopes in fossil brachiopods shows that it was probably no longer than 0.5 to 1.5 million years. [41] The event was preceded by a fall in atmospheric carbon dioxide (from 7000ppm to 4400ppm) which selectively affected the shallow seas where most organisms lived.
The "ago" is implied, so that any such year number "X Ma" between 66 and 145 is "Cretaceous", for good reason. But the counter argument is that having myr for a duration and Mya for an age mixes unit systems, and tempts capitalization errors: "million" need not be capitalized, but "mega" must be; "ma" would technically imply a milliyear (a ...
James Hutton based his view of deep time on a form of geochemistry that had developed in Scotland and Scandinavia from the 1750s onward. [6] As mathematician John Playfair, one of Hutton's friends and colleagues in the Scottish Enlightenment, remarked upon seeing the strata of the angular unconformity at Siccar Point with Hutton and James Hall in June 1788, "the mind seemed to grow giddy by ...
In 1862, the physicist William Thomson, 1st Baron Kelvin published calculations that fixed the age of Earth at between 20 million and 400 million years. [19] [20] He assumed that Earth had formed as a completely molten object, and determined the amount of time it would take for the near-surface temperature gradient to decrease to its present value.
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