This study investigates and interprets the distinctive textures of agates in the latest Triassic North Mountain Basalt. Ten representative samples were investigated in terms of petrography, mineralogy and mineral chemistry. Agate veins typically show three zones, elsewhere interpreted as forming in situ from an initial impure amorphous silica sol by self-organisation processes. (1) An outer zone of microgranular quartz and irregular goethite flames; (2) banded chalcedony; and (3) a central zone of macrocrystalline quartz. Horizontal veins show asymmetric development of these zones and vertical veins show deformation of flames. Some agates are brecciated with clasts cemented by zeolite and agate. The North Mountain Basalt comprises >150 m thick lower and upper ponded flow units and a middle unit of multiple thinner flows. The base of the lava pile is saucer shaped. Hydrothermal circulation driven by heat from the lower flow unit dissolved halite from the underlying Blomidon Formation. The saline waters were hot enough (>300 °C) to dissolve silica and iron, in part from the cooling lava flows. The resulting silica sol migrated up dip to the southern margin of the basalt, where actively deforming faults along the Minas Fault Zone provided pathways and accommodation. Heat from the thick upper flow unit re-activated hydrothermal flow through the Blomidon Formation and mixed with zeolite-rich fluids, producing agate with an outer rim of zeolite, in some cases reactivating or fragmenting earlier agate veins. The particular setting of these agate veins demonstrates linkage between observed textures and interpreted details of hydrothermal circulation.