IGNEOUS PROVINCES IN WESTERN UNITED STATES 



565 



mentary matrix. Lavas are analcite basalt. Feldspar is scarce or absent 

 and analcite abundant. MgO, CaO, and Na a O are high; K 2 is low. 



The Navajo volcanic field (Williams, 1936) consists principally of a 

 number of necks of tuff breccia and agglomerate crowded with frag- 

 ments of granitic rocks. These breccias and agglomerates are high in K 2 

 in contrast to the Hopi Ruttes rocks, and fairly low in Na 2 and fairly 

 high in MgO and CaO, and have been called sanadine-rich trachybasalts 

 and leucite basalts. Williams suggests that an originally sodic ultrabasic 

 magma having the composition of nepheline basalt reacted with the 

 potash feldspar of granites in the basement and so attained the high 

 potassic composition which prevails in the subprovince. 



In the interior of the Plateau, in the laccolithic mountains, soda greatly 

 exceeds potash. The same is true in the Hopi Ruttes field along the 

 southern edge of the Colorado Plateau, but in the intervening Navajo 

 field potash greatly exceeds soda. 



The Elkhead Mountains of northwestern Colorado constitute a high 

 alkalic volcanic field. The suite is unusual with rocks containing both 

 olivine and quartz, a nepheline-bearing trachyte with phenocrysts of yel- 

 low-brown mica in a groundmass of sanadine and nepheline, and analcite 

 basalt without feldspar, and with dikes of soda verite, analcite syenite, 

 and soda syenite (Carey, 1955). 



Central Wyoming. Leucite Hills are located in south-central Wyoming 

 on the north end of the Rock Springs uplift. They are remnants of lava 

 flows and cinder cones on a mid-Tertiary erosion surface, now much dis- 

 sected and left about 800 feet above the present valley floors. The rock is 

 called Wyomingite, and contains phlogopite, leucite, and diopside ( Cross, 

 1897). 



The Rattlesnake Hills field of central Wyoming consists of three large 

 necks and a number of small necks and related dikes in an area of 150 

 square miles. The first and largest intrusions and extrusions were viscous, 

 acid quartz latites. Following these a series of highly alkalic trachytes, 

 phonolites, and vogesites were erupted. (Vogesites are lamprophyres, 

 ' generally considered to be hypabyssal. ) The alkalic rocks are unique for 

 their content of the relatively rare feldspathoidal minerals, huayne, and 

 nosean. Although the necks are in a rather small area, the amount of 



material ejected was large and certain clastic parts are believed to have 

 been transported 100 miles from the volcanic center. The activity is dated 

 as mid-Eocene (Carey, 1954). Most of the immediate ejecta has since 

 been eroded away, but water-transported fragments are prominent in a 

 middle and upper Eocene formation of die general region (Van Houten, 

 1955). 



Black Hills. Across the north end of the Rlack Hills uplift is a row of 

 imposing Tertiary volcanic necks and laccoliths in Mesozoic strata known 

 from west to east as Devils Tower, Rear Lodge Mountain, Rear Rutte, 

 Inyankara Mountain, and Mineral Hill. These are composed of phonolite, 

 pseudoleucite porphyry, nepheline syenite, and aegerite syenite (Robin- 

 son, 1956). 



Several of the centers of Tertiary igneous activity are domal uplifts in 

 the Paleozoic and Mesozoic sedimentary rocks and the underlying cause 

 of doming is regarded by Noble et al. (1949) as due to the intrusion of 

 stocks rather than laccoliths. One of the domes includes the noted Home- 

 stake gold mining district at Lead. It is 10 by 12 miles in size and con- 

 tains several rather ragged Tertiary stocks and numerous sills and dikes. 

 The intrusive rocks have been described as phonolite porphyry, rhyolite, 

 and quartz porphyry ( O'Harra, 1933 ) . 



The entire domal structure of the Rlack Hills, some 50 miles by 1(K) 

 miles, is considered possibly due to a major Tertiary batholithic intrusion 

 by Noble et al., but they see no way of finding evidence of the intrusion. 

 The gravity picture which might help is clouded by the dominance of 

 gravity lows over the adjacent Cretaceous and Tertiary basins. 



Central Montana. North-central Montana is characterized by a num- 

 ber of mountain groups, each of which owes its existence to igneous 

 activity, both intrusive and extrusive. The region is east of the Laramide 

 belt of intense compression and the magmas have penetrated nearly 

 horizontal sedimentary strata. 



The rocks range from rhyolites to basalts in one category and from 

 shonkinites through nepheline syenites to syenites in another. The rocks 

 of the latter category are rich in potash and soda and almost devoid of 

 plagioclase. The rocks of each mountain group fall into one or more 

 eruptive stages; and the rocks of each stage have peculiar mineral and 



