AVB. They are chemically bimodal, consisting mainly of mafic and felsic lavas with almost no intermediate lavas having been verified. The shield volcanoes share a similar evolution, having formed by an early stage of predominately voluminous felsic volcanism and a late stage of relatively minor mafic volcanism. Although the felsic lavas produced during the early evolutionary stage were high in silica content, they remained fluid enough to build felsic shields. The volcanic fields consist of lavas that range from basanites and trachybasalts to evolved trachytes and phonolites. They comprise several small volcanoes that are in the form of cones, domes and plugs, most of which likely experienced episodic and short-lived activity. In contrast, the few larger structures in each volcanic field experienced multiple eruptive events over a longer timespan. Both fields were active contemporaneously throughout much of the Pleistocene. Shield volcanoes The Rainbow Range is a diameter, moderately dissected shield volcano that was active 8.7 to 6.7 million years ago. Four volcanic episodes characterized by highly fluid alkaline and peralkaline lava flows created an thick assemblage exposed on the north flank. A basal sequence of comenditic trachyte flows are unconformably overlain by flows and flow breccias of mugearite which in turn are overlain by a thick sequence of columnar-jointed comendite flows. Late stage volcanism resulted in the creation of scattered hawaiite dikes, plugs and minor capping flows over the north flank. Accompanying the northeastern flank is Anahim Peak, a trachyte plug surrounded by a pile of hawaiite flows that are four to eight times thicker than those of the Rainbow Range. The abnormal thicknesses of the Anahim Peak hawaiite flows coupled with a significantly coarser-grained groundmass and higher percentage of phenocrysts than the upper portions of the flows suggest that they were ponded during eruption, possibly as lava lakes within a former cinder cone. To the east lies the Ilgachuz Range, a slightly smaller and less deeply dissected shield volcano that formed 6.1 to 4.0 million years ago. Its nearly circular and gently sloping structure, in diameter, consists of four assemblages that were deposited in six major episodes of activity. The lower shield assemblage represents the oldest exposed rocks in the Ilgachuz Range. It comprises pyroclastic breccias, domes and lava flows of alkali rhyolite that have been hydrothermally altered. The overlying upper shield assemblage, which forms the bulk of the Ilgachuz Range, encompasses a series of comendite and pantellerite flows