The dikes were classified under five field types, which in the order of their intrusion are: (1) amphibole phonolite, (2) pseudoleucite mafic phonolite, (3) augite mafic phonolite, (4) biotite phonolite, and (5) syenite porphyry. The rare amphibole phonolite dikes appear to be related to the quartz latite volcanics. The other four types form a gradational series related to the mafic phonolite volcanics. Types (2) and (3) constitute more than half the dikes of the area, and type (4) approximately one-fourth. Syenite porphyry and a few dikes of nepheline syenite, monzonite, and alnoite constitute the remainder.

The dikes vary in width from less than 2 feet to more than 20 feet. Few segments can be traced continuously for as much as a mile. Dikes are most numerous north and east of the mountains, where a radial trend out from the mountains is clearly recognizable. Fractures into which the dikes were intruded were produced by a centrally acting force and developed outward from the central part of the area.

An irregular intrusive body composed of alnoite and monticellite peridotite containing 32 per cent of monticellite was mapped 3 miles northeast of Geraldine.

A new term, headed dike, is proposed for intrusive bodies like the one found 8 miles southwest of Geraldine. This body is linear in plan and bulbous in cross section narrowing downward to a feeder dike. The top contact is conformable with the bedding of the Cretaceous sandstone. Fergusite and shonkinite are the principal rocks of the headed dike. Differentiation of the two types was gravitative after intrusion.

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