Named after Mount Ainslie, Australian Capital Territory where their type location occurs, the Ainslie Volcanics are composed of Dacitic ignimbrite and minor volcaniclastic and argillaceous sediments. The lithology is bluish grey dacitic tuff, which can be massive or foliated, also dacitic agglomerate and shale. These volcanics contain chloritised cordierite and some have red almandine garnet. Jasper is found on low hills on the north side of the Molonglo River. The thickness is at least 700 metres. The magma was formed by melting an aluminium-rich pelitic sediment. The eruption came from a volcano into shallow sea water. The deposits built above sea level as they progressed. The underlying sediments now make up the Canberra Formation. Between Hall and Namina Hill on Spring Range, the Mount Painter Volcanics lie unconformably on top.