Thanks to the natural resources of the country, every American, until quite recently, could look forward to making more money than his father, so that, if he made less money, the fault was his; he was either lazy or foolish.
スラッシュは、カンマ、セミコロン,前置詞の前に入れます。
Thanks to the natural resources of the country, /every American, /until quite recently, /could look forward/ to making more money than his father,/ so that, /if he made less money,/ the fault was his; he was either lazy or foolish.