The Ordovician quartz-rich greywackes of the Snowy Mountains region are essentially mixtures of two components. These are quartz and clay minerals. The chemistry of any rock is largely determined by its proportions of clay minerals so that there is a strong negative correlation between Si and those elements, other than Si, contained in clay minerals. Elements that occur only in feldspar, but not in clay minerals, that is Ca, Na, Sr and Pb, have a low and variable concentration. These four elements have even lower abundances in the Silurian greywackes deposited in basins on a basement of the Ordovician rocks. This is interpreted to have resulted from a further cycle of chemical weathering that acted on Ordovician source rocks to produce the Silurian detritus. In a similar fashion, the Ordovician rocks are thought to have formed by weathering of more feldspar-rich sedimentary rocks at the margins of the Lachlan Fold Belt. Two episodes of chemical weathering are therefore recognised. The first produced the feldspar-poor Ordovician rocks; the second acted on part of these to form the almost feldspar-free Silurian strata. In both cases, Ca, Na, Sr and Pb were lost as feldspars were converted to clay minerals.