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Rubidium and strontium are used as tracers in the history of differentiation of the continental sial, and for this purpose it is assumed that the Rb/Sr ratio, follows the aggregate enrichment of sialic components sufficiently well to represent an approximately linear measure of the entire process. Measurements of the Sr•/Sr •6 ratios in basalts and other igneous rocks of subsialic origin suggest that the source regions of sial have had a SrS7/Sr• ratio in the range 0.705 to 0.710 in different areas and at different times in the span of geologic history. Measurements of the Rb/Sr ratio in sialic rocks of the continental crystalline basement have indicated that the weighted average value for this ratio is in the vicinity of 0.25. This means that the SrS*/Sr•6 ratio in average sialic rocks of the continental basement would increase at a faster rate than that in the subsialic source regions, by an amount equal to approximately 0.010 per billion years. This is more than five times the standard deviation precision error in measurement. A comparison of the ratio SrS7/Sr • with the geologic age of the rock, in samples of typical sialic basement rocks leads to the conclusion that there has been a continuous generation of primary sial from subsialic source regions that has caused the continental areas to grow roughly in proportion to the extent of the geological age provinces. In North America this areal growth is estimated to be at the average rate of about 7000 kmym.y., and it seems to have been operative over most of geologic time. A model is formulated in which the proportion and age of reworked material that is incorporated into new sialic basement are related to the SrS*/Sr• development in the various materials involved. The model is generalized with all the parameters left open. An example set of values for the various parameters is tested on the model to indicate where there is need for more definitive data. This example set is derived from data available at present which, although very limited, indicate clearly that if the reworked material is young enough it can constitute a major proportion of a new sialic basement area, but if it is ancient (1 to 2 b.y. in primary age prior to incorporation) it could not be present in more than a minor proportion of the typical new basement. The abundance of SF * is related to the abundance of its parent Rb 8' in the materiM and to the time since separation of the rocks from a source at depth. We believe that the pair of elements Rb-Sr is better suited for this study than the pair U-Pb because of the great geochemical dissimilarity of the former. As a result Rb and Sr are more efficienfiy separated from each other than U and Pb during the dif-1 M.I.T. Age Studies, No. 36.ferentiation of sialic crust from its source regions. It is granted that the long half-life of Rb •7 does not create the magnitude of change in the Sr • abundance that is produced in the radiogenic isotopes of lead and that measurements of variations are therefore less precise. Nevertheless, we are currently co...