<Tr> <Td> CaO </Td> <Td> 6.4 </Td> </Tr> <Tr> <Td> MgO </Td> <Td> 4.7 </Td> </Tr> <Tr> <Td> Na O </Td> <Td> 3.1 </Td> </Tr> <Tr> <Td> Fe as FeO </Td> <Td> 6.7 </Td> </Tr>

Which of these is not a layer of the earth