Nielsen., Niels. “Martin R. Huberty & Warren L. Flock: Natural Resources. McGraw-Hill, New York, London. 1959. 24 X 16 Cm. 556 S. Pris 85 Sh. 6 D”. Geografisk Tidsskrift - Danish Journal of Geography, vol. 58, Jan. 1959, https://tidsskrift.dk/geografisktidsskrift/article/view/46444.