Equations are derived which link the content of relatively insoluble components Si, Al, Fe and K in source rock and derived alluvial sediment. The equations also involve the ratio of specific suspended load to dissolved load and specific bed load to suspended load (Tb/Ts) for the rivers which deposited the sediments. These ratios are generally unknown, but by analogy with modern rivers are taken to occupy the ranges and 0 ≦ Tb/Ts≦1. This is sufficient to put close limits on the possible composition of the source rock. The equations suggest that clastic sediment forming the Late Proterozoic Torridon Group of Scotland was derived from sediments or metasediments, rather than from average upper continental crust, granitoids or gneisses like those presently exposed nearby.

Mass balance equations are derived which link the ratios Ts/ (suspended load/dissolved load from chemical weathering) and Tb/Ts (bed load/suspended load), with any two geochemical components present in the source rock and the alluvial system. If the dissolved load is unknown the ratios can be estimated from the relatively insoluble silica and alumina. The ratio Ts/, which for large river basins depends on climate and relief, can thus potentially be determined from ancient alluvial sequences.

The equations help define the source composition of a group of 13 modern rivers for which Ts, and alluvial geochemistry are known. These rivers together drain 27% of the continental surface. For a source area with the average continental sandstone to shale ratio of 0·6 the observed average value of Ts/ is obtained when limestone, sandstone and shale are present in the proportions 6·7:21·6:35·7. The figure of 64% sediment in the source area is very similar to the 66% determined by Blatt and Jones (1975) from geological maps of the continents. The equations also show that average bed load transport rate into these 13 basins is about 27% of total transport, and into the Amazon basin about 37%. Bed load transport rates out of the basins, into the sea, are relatively very small.