“In a third class of cases,” Lord Rayleigh points out, “the uranium mineral, pitchblende, occurs in a metalliferous vein, and the lead isotope produced in the mineral is diluted with common lead which entered into its original composition, ... but the complications cannot, I think, be considered to modify the broad result. “A determination of the amount of helium in minerals gives an alternative method of estimating geological age; but helium, unlike lead, is liable to leak away, hence the estimate gives a minimum only. I have found in this way ages which, speaking generally, are about one-third of the values which estimates of lead have given, and are, therefore, generally confirmatory, having regard to leakage of helium.” Dr. Homer P. Little, of the National Research Council, Washington, D. C., tells us (_Scientific American Monthly_, August, 1921, p. 173) that “from both calculation and experiment it is found that one gram of uranium will produce helium at the rate of one cubic centimeter in 9,600,000 years. The ratio between the amount of radium in a mineral and the amount of helium present therefore allows us to calculate the age of the mineral. The amount of uranium originally present compared to that left does not enter into the problem unless extreme lengths of time are under consideration, because of the fact that it is calculated to take 5,000 million years for one-half a given volume of uranium to disintegrate. “It is perfectly true that much of the helium generated may escape. The assumption is, however, that in some minerals comparatively little escapes: zircon, particularly, seems to be an effective retainer. This mineral shows very effectively the increasing ratio of helium to uranium as consecutively older rocks are examined. Recent or Pleistocene specimens from Vesuvius show an apparent age of 1 million years; Miocene specimens from the Auvergne, France, of 6.3 million. The Devonian of Norway furnishes specimens 54 million years in age, and the Upper Cambrian of Colorado specimens of 141 million years; the Archaean of Ceylon, of the diamond-bearing rocks of South Africa, and of certain rocks of Ontario furnish specimens aged 286, 321 and 715 million years, respectively.” The following table gives the mean of the results of Professors Boltwood and Holmes’ careful studies, based upon the accumulation of lead as a final product of the uranium series: MILLIONS OF YEARS Carboniferous 340 Devonian 370 Pre-Carboniferous 410 Silurian or Ordovician 430 Pre-Cambrian: Sweden 1,025 United States of America 1,310–1,435 Ceylon 1,640