But estimates based on the lead ratios of radio-active minerals offer results consistent among themselves. That is, whenever fresh, primary, uranium-bearing minerals of the same geological age have been examined, the lead ratios are always found to remain constant. The value of the ratios increases or decreases as the geological age of the respective mineral increases or decreases. In other words, the lead ratios are in keeping with the geological age. The procedure of applying the lead ratio in calculating geological time can be briefly stated thus: The rate of production of lead from uranium can be readily calculated. The rate at which helium is generated is accurately known and the quantity of lead liberated in the same time is approximately 6.5 times that of helium. In a year one gram of uranium produces 1.25 × 10⁻¹⁰ grams of lead, and at this rate 8,000[2] million years will be required for the production of one gram of lead. [2] A more recent and accurate computation reduces this 8,000 million years to 7,500 million years. There is no serious difficulty in applying this method for measuring geologic time, except that it is necessary to determine whether the lead is of radio-active origin or original lead. The presence of original lead is likely to mar the constancy of the lead ratio essential for accurate results. But ordinary lead need not be confused with uranium lead, as the atomic weight of ordinary lead is 207.1 and that of uranium lead is 206.2. Values between these two figures imply a mixture of two types of lead. For reliable calculations, a series of fresh, primary minerals of the same geological age showing a constant lead ratio of atomic weight 206.2 needs to be examined. The following table shows the geologic time that has elapsed between the first evidences of life and the present, as calculated by Barrell from radio-active data. The figures are his minimum and maximum estimates. It will be noticed from the figures in the table that the earliest life of which we have fossil records is about 1,500,000,000 years old. From this, it could be safely concluded that the inception of life on earth must have taken place much earlier. It is quite significant that each geological era, occasionally a geological period, has its characteristic grouping of life developed from the life of preceding periods. As we climb higher in the geological column, life becomes more and more complex and specialized. From the one-celled life of the Archeozoic it passes through the invertebrates—fishes—amphibians—reptiles—birds and mammals to man of the Recent time. Since Barrell’s publication of the estimates of geologic time as measured by means of radio-activity, some further studies have been made along the same line, but, as no generally accepted results have shown any marked differences from Barrell’s results, it has been deemed advisable to use his age data as perhaps our present most adequate guide as to the length of geologic periods.