During the last three decades, a number of radio-active changes of one chemical element into another have been discovered and studies of certain minerals and rocks containing various radio-active elements have created means to calculate their age with remarkable accuracy. “A study of the various radio-active elements contained in minerals and rocks,” says Harker, “has shown that it is possible, in certain favorable cases, to calculate directly their age in years.” The radio-active minerals are commonly found in igneous rocks. They are widely distributed all over the world. The parents of the whole series of radio-active elements are uranium and thorium. They possess the highest atomic weights of all known elements. Each of these parental elements transforms itself through a succession of changes. The final product of uranium is the formation of the metal lead and the gas helium. These transformations take place in one direction only, that is, from an element of higher atomic weight to an element of lower atomic weight. It has also been demonstrated beyond question that these transformations are unalterable by any process whatsoever and that they are independent of temperature, pressure or any other physical or chemical state. Temperatures up to 2,500 C. and pressures up to 600 tons per square inch have not been found to influence the rate of transformation. Time estimated on the basis of these processes, therefore, offers a more reliable result than that obtained by any other method hitherto known. Detailed descriptions of how the metal uranium slowly and regularly breaks down in a descending series into the metal lead and the gas helium, will be found in the literature on radio-activity. For our purpose, it suffices to say that according to Barrell, an atom of uranium which breaks up will ultimately give rise as a stable product to eight atoms of helium and one atom of lead. Since the rate of transformation is known, data for calculating the age of the mineral and with it the rock formation of which it is a part, can be obtained by measuring the quantity of helium and lead in the rock and comparing it with the quantity of uranium in the same volume of material. But, as helium is a gas, it is likely that a certain portion of it leaks out and consequently the estimate of age on the basis of how long helium had been in contact with uranium and lead is to be regarded as a minimum estimate. For example, the age of the mineral thorianite that occurs abundantly in the sands and gravels of Ceylon has been estimated to be 280,000,000 years, but the mineral is doubtless much older, as, ever since it was broken away from its original home in the pegmatite dikes of Ceylon, it lay exposed to the action of weathering and it was, therefore, very likely that during all these years a certain percentage of its helium contents had leaked away.