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• The second rock type in the existing and Phase IV VLF area is Tertiary aphanitic <br />phonolite, most frequently occurring as narrow, often discontinuous dikes intruding the <br />granodiorite in a northeasterly direction. Phonolite dikes generally are blue-gray in color <br />(depending on degree of alteration), porphyritic with low percentages of K-feldspar <br />phenocrysts set in an aphanitic groundmass and may range from massive to fissile in <br />character. Dikes typically range in width from 0.5 foot to 10 feet. Phonolite also occurs <br />as a plug that forms the knob called Beacon Hill. At the surface, the Beacon Hill Plug <br />measures approximately 2,000 feet long in anorth-northeasterly direction and 1,200 feet <br />wide. It is interpreted to taper considerably downward below the 8,400 foot elevation <br />and is much the same as the dikes described above in terms of composition and texture. <br />The third rock type in the existing and Phase N VLF area is the lapelli breccia. <br />Based on the geotechnical investigations performed on the breccia occurring on Beacon <br />Hill, it is weathered to an average of about three feet below the ground surface. More <br />• specifically, the slopes of Arequa Gulch are covered with a thin layer of colluvium and <br />soil. Bedrock outcrops at the surface have been noted but are not dominant. No notable <br />alluvial deposits are present within the footprint of Phase N. <br />Mapping in the Arequa Gulch area for the existing VLF indicated the existence of <br />four important structural orientations. The strongest trend is about N20E and is in the <br />central portion of Arequa Gulch. The lineaz features are small faults or shears consisting <br />of narrow clay gouge seams (0.1 to 1.0 feet in width), surrounded by five- to eight-foot <br />wide envelopes of highly fractured, oxidized, and weakly clay-altered granodiorite. A <br />less strong lineation [rends N70E primarily along the northem portion of the Arequa <br />Gulch site. These lineations are schistose shear zones and range up to five feet in width. <br />The other lineations aze at a N40-SOE structural orientation delineated by discontinuous <br />phonolite dikes that increase in frequency to the north, and a N45W trend of <br />discontinuous, narrow faults observed only in the northem portion of the site. There <br />appears to be minimal displacement along faults. <br />• <br />19 <br />