Judith Mountains

The Judith Mountains are formed of a series of coalescing dome-shaped uplifts caused by the intrusion of a series of laccoliths and sills into a conformable sequence of marine sedimentary rocks of Cambrian to Cretaceous age. There are six principal laccoliths, some 16 subordinate laccoliths, and three cross-cutting intrusions within the mountains. The principal rocks of the laccoliths are granite porphyries which range from aegirine-bearing to hornblendic varieties, and syenite porphyries containing aegirine-augite, although their abundance and distribution are not well known. Syenites are present mainly as small stocks among which varieties with a greenish diopside are to be found. There is a transition of the syenitic rocks into dioritic types. Nepheline syenites, tinguaites and phonolites are abundant in the northern part of the area as sills within the sediments and as dykes within the major intrusions. They all comprise alkali feldspar, interstitial nepheline, pyroxene, which is usually zoned from diopside through to aegirine, analcime or sodalite in some of the coarser rocks and zeolite and melanite in some varieties. Many of these rocks are crowded with large alkali feldspar phenocrysts generally 0. 5-10 mm but up to 20 cm in length, the petrogenetic significance of which are discussed by Wallace (1956).