Boltwood is credited as being the first to introduce a uranium-lead dating technique to determine the age of geological samples. The discovery stemmed from his investigations of the uranium decay series and conversations with Rutherford. In 1904, Rutherford gave lectures around the topic of radioactivity as a tool for geologic dating, and presented calculations based on the presence of helium as a product. He dated a sample of fergusonite at 40 million years, with the caveat that helium could escape and would therefore only provide a minimum age. The following year, Boltwood made the assertion that lead was the final decay product in the disintegration of uranium, and that Pb:U ratios increase in older geological samples. In 1907, he published results of analyzing ten mineral samples from different world locations, including a thorianite that measured 2.2 billion years old. This value was ten times greater than any previous estimated age of the Earth, and geologists did not immediately accept the validity of radioactivity as a dating method. Boltwood published a single paper on radiometric dating, instead focusing the majority of his research on the uranium decay series.