What is the principal that states that younger rocks lie above other rocks?

The principal that states that younger rocks lie above other rocks in the superposition principal.

Tropical climate is experienced in the nothern countries of south america

The us is first  the  u.s. economy  remains the  largest in the world  in terms of nominal  gdp. however, the  u.s. economy  loses its spot as the number one  economy  to china when  measured  in terms of  gdp  based on ppp. in these terms, china's  gdp  is $23.19 trillion exceeds the  u.s. gdp  of $19.42 trillion