AbstractIt is shown that the high‐solubility layer previously demonstrated (Clelland, Cumming&Ritchie, 1951) on silicious dust‐particle surfaces is not a surface layer of hydrated silica; and evidence is adduced that it is a vitreous silica layer of the Beilby type, formed during the production of the dust by crushing and grinding. An important part of the evidence is the demonstration that prolonged grinding of quartz leads to a reduction in density. This reduction is attributed to partial conversion of the dust particles to vitreous silica: no conversion to other crystalline modifications of silica could be detected. Further, when the high‐solubility layer has been removed from silicious dust particles by solvents, it can be regenerated by a purely mechanical polishing process. The regenerated layer yields silica into aqueous media very largely in colloidal