60 BASIC AND ULTEABASIC IGNEOUS ROCKS— BENSON. IMBMOIRS [v : oL. , xix I ; 



THE WESTERN CORDILLERA OF NORTH AMERICA. 



Blackwelder's ('13) convenient sketch-map of the Rocky Mountain orographic elements 

 and Idding's ('13) summary of the distribution of igneous rocks therein allow us to comment 

 briefly on the tectonic surroundings of the various types of igneous rocks developed, the charac- 

 teristics being indicated more clearly if we do not confine our attention to basic and ultrabasic 

 rocks only. East of the Rocky Mountain zone there is a region of laccolitic hills rising with local 

 upturning of the strata from an almost unfolded terrain. Commencing in Arkansas, where 

 the series to be described is linked with the complex of Magnet Cove, we have the nepheline- 

 syenites and associated alkaline rocks of the Fourche Mountains, probably of late Cretaceous age; 

 nepheline-syenites in the Apache Mountains of West Texas; the strongly alkaline complex of 

 Pikes Peak on the eastern flank of the Front Range; the post-Cretaceous alkaline laccolites of 

 the Black Hills of South Dakota and Wyoming, containing mostly rocks of a moderate basicity, 

 but also strongly basic rocks (ijolite, nephelinite, fourchite, etc.) ; and the laccolitic region of 

 Central Montana, in which the alkaline facies is not always so strongly developed, though very 

 distinct in the laccolites and stocks of the Highwood and Bearpaw Mountains. Among the 

 first of these is the well-known gravitationally differentiated laccolite of Shonkin Sag. A 

 monzonitic series occurs in the Little Belt Mountains, and a mixed facies of diorites and gabbros 

 associated with shonkinites and other alkaline types in the Elkhorn and Crazy Mountains. In 

 the last region the alkaline rocks form a marginal zone of intrusions about the central stock of 

 subalkaline rocks, which invade upturned Eocene sediments. This recalls the distribution of the 

 subalkaline intrusions in areas of local compression and the regional development of rocks with 

 a "subdued alkaline character" in Skye and Mull. (See p. 13.) Alkaline rocks also occur in 

 the Kootenay region and continue across the border into Canada. 



West of this zone there is the continuous zone of open folds and faults extending from 

 Yellowstone Park to Colorado. In these the petrographic characters of the rock are less uniform. 

 In Yellowstone Park andesitic laccolites were formed at the end of the Laramide orogeny and 

 later basaltic volcanoes have cores of gabbro and diorite. Among the effusive rocks of the 

 Tertiary period alkaline and subalkaline types occur. They are also present in flows and dikes, 

 accompanying generally monzonitic or dioritic stocks in Colorado. In the Front Range and 

 the Sangre de Cristo Mountains the pre-Cambrian schists, etc., are invaded by dikes of granite, 

 monzonite, diabase, gabbro, pyroxenite, and peridotite, and there is also a large gabbro batholith 

 (Animas Valley). 



To the west of this we have in the south the vast plateau of Utah, with laccolitic intrusions 

 generally of andesitic characters, but with gabbros, diorites, etc., and only exceptionally strongly 

 sodic types. To the north is the region of compressed folds and overthrusts in western Montana 

 and Idaho, where there are the subalkaline Idaho and Boulder batholiths of quartz-monzonite 

 with granitic, dioritic, and gabbroid facies and the quartz-diorite of Marysville. The intrusion 

 of these probably followed the commencement of Laramide folding. 



We turn now to the western portion of the cordillera. The northernmost mass of ultra- 

 basic rock of which the writer possesses information is that described by Martin and others 

 ('15) in the Kenai Peninsula of Alaska, where peridotite invades a highly disturbed series of 

 Paleozoic or possibly Mesozoic greywackes, cherts, limestones, and basic igneous rocks. In the 

 less metamorphosed terrain in the southwestern portion of the peninsula there are Triassic 

 limestones and contorted cherts with pillow-lavas. 



Southward from this stretches the Coast Range batholith for nearly 850 miles, with a 

 width of about 60 miles in one place. It is made up of a number of separate batholiths ranging 

 in age from early Jurassic to early Cretaceous. It varies in composition from gabbro to acid 

 granite, but the prevailing type is a granodiorite usually massive, but often with a primary 

 gneissic structure. McConnell ('13) and Bowen ('15) state that the smaller outlying intrusions 

 to the west of this in the Prince of Wales Islands are of gabbro. Norite and gabbro also appear 

 as marginal facies in Queen Charlotte Sound, and orbicular gabbro is recorded. Hornblendite 

 locally forms a marginal ultrabasic modification (Bancroft, '13). The features here described 



