Note 2 — Expedition to the Mines of the Igorrotes

George F. Becker in his “Report on Geology of the Philippine Islands”—in Twenty-first Annual Report of U.S. Geological Survey (Washington, 1901), part iii, pp. 487–625—cites (p. 622; cf. also p. 517) the geologist R. von Drasche thus: “Layers of tuff [or tufa—a volcanic rock formed of agglutinated volcanic earth or scoria] are also exposed (Fragmente zu einer Geologie der Insel Luzón, pp. 29–31) at many points between Aringay and Benguet, but these tuffs toward the interior, even at Galiano, are ‘no longer earthy, but quite hard, crystalline, and sandstone like.’” This probably explains Martin’s description of the hard ground.