Weathering produced a total of 2--4¿1025 g of carbonates, clay minerals and SiO2 (chert and quartz) in approximately equal proportions during the history of the Earth. This amounts to approximately 1% of the mass of the mantle. A third was produced during the Hadean, before 3900 Ma. The amount of carbon consumed is greatly in excess of the crustal carbon reservoir and is equivalent to the total amount of carbon in the upper mantle. It is concluded that most of the material produced was subducted and that only a small fraction is preserved in sedimentary rocks. The chemistry of the subducted sediments was highly diverse in response to the variability of the climatological and depositional environment. This resulted in significant mantle heterogeneities on different scales. The absence of Hadean sedimentay rocks is explained by postulating that all the sedimentary material produced in this period was subducted after being mixed with a large amount of basaltic and komatiitic impact debris derived from the intense meteorite flux. The meteorite flux diminished to 0.2--0.4 km3/yr at the Hadean--Archean boundary, which resulted in a large reduction of the amount of impact debris, and sediments began to accumulate at the earth's surface. At the same time copious amounts of felsic intrusive rocks, derived from silica-enriched subducted material, were produced and a continental cryst began to grow rapidly. The model indicates that the oldest sedimentary rocks contain a substantial proportion of impact-derived material and that careful investigation of these rocks may provided support for the model. ¿ American Geophysical Union 1988