These results, a total of 1,400,000,000 years, greatly transcend Lord Rayleigh’s (Strutt’s) earlier calculations regarding the antiquity they assign to Paleozoic and Pre-Cambrian times. In 1918, Prof. Joseph Barrell reviewed the various methods employed and the results obtained in the attempt to determine from geological, chemical and physical evidences the time that has elapsed since the beginning of the Cambrian Period (when abundant fossil invertebrates are first met with), and reached the following time estimates for the principal divisions of the geologic record (exclusive of the Pre-Cambrian rocks): Cenozoic time,   55,000,000 to  65,000,000 years long Mesozoic time,  135,000,000 to 180,000,000 years long Paleozoic time, 360,000,000 to 540,000,000 years long The time thus established covers a period of from 550,000,000 to 700,000,000 years, or from ten to 15 times longer than has usually been accepted by geologists. Pre-Cambrian time was found to have a similar order of magnitude; but here the evidence rests largely upon the radioactivity of the crystalline rocks formed during this vast period. It is now universally accepted that the time required for the formation of the Pre-Cambrian rocks was fully as long as, if not longer than, that for the succeeding geological divisions. The Archaean deposits have a vertical thickness, in the regions north of the Great Lakes, estimated at about 65,000 feet, or 12 miles. Their base, as a matter of fact, has never been reached. It is interesting to note that the granites of Norway, Canada, Texas and East Africa have an indicated age of 1,120,000,000 years, measured in terms of radium products. Prof. Henry Norris Russell, of Princeton University, concludes, from his careful investigations in radioactivity, that the age of the earth is “a moderate multiple of 1000 million years.” Professor Joly has computed that if there are two parts of radioactive material for every million million parts of other matter throughout the whole volume of the earth, and this is considerably less than he has found on the average in the earth’s crust, then this earth, instead of cooling off, is actually now heating up, so that in a hundred million years the temperature of the core will have risen through 1,800 degrees centigrade. Dr. Millikan observes (_Science_, July 9, 1921) that this is a temperature “which will melt almost all of our ordinary substances.... It means that a planet that seems to be dead, as this our earth seems to be, may, a few eons hence, be a luminous body, and that it may go through periods of expansion when it radiates enormously, and then of contraction when it becomes like our present earth, a body which is a heat insulator and holds in its interior the energy given off by radioactive processes, until another period of luminosity ensues.”