John Joly

In 1899, Joly calculated the Earth's age by estimating how long it would take for rivers to deliver enough dissolved sodium to reach the ocean's current salinity levels. His method yielded an age of roughly 100 million years, which was a serious underestimate because he did not account for salt removal mechanisms like evaporite deposition. Nonetheless, it was one of the first quantitative, physics-based attempts at dating the Earth.

Joly spent his academic career at Trinity College Dublin, where he served as a professor of geology and mineralogy. He worked there for decades until his death in 1933, and he is remembered as one of Ireland's most prominent early 20th-century scientists.

Yes, Joly was a prolific inventor and researcher. He developed a method for estimating the radiation dose in cancer treatment called the "Joly irradiation method" and also pioneered the use of color photography in geology, photographing thin sections of rocks through a microscope.

Joly's estimate of approximately 100 million years was far below the modern accepted age of about 4.54 billion years, which was later established through radiometric dating of meteorites. His sodium-based method could not account for the vast recycling of oceanic crust and chemical sinks that remove salt from seawater.