In connection with investigations of horizons with volcanic ash in Swedish peat-bogs (C. Persson 1966) three Icelandic dacitic to rhyodacitic ash layers have been investigated. These are Askja 1875 A.D., Hekla III about 870 B.C., and Hekla IV about 1880 B.C. Information from the literature is compiled, and some new chemical analyses given. Some trace elements studies have been made and the refractive index of the glass determined. Pumice from Askja 1875 A.D. was studied in thin section. Particle size in relation to distance from the volcano was examined. The chemical analyses show that the volcanic glass changes composition with increasing age forming hydrous aluminosilicates such as kaolinite and sericite.