462 KEMP AND BILLINGSLEY SWEET GRASS HILLS, MONTANA 



the iron ores were discovered which were described by Dr. A. E. Ledoux. 

 Although once opened up by shafts and some drifting, the old, abandoned 

 excavations have become inaccessible since Dr. Ledoux's visit in 1890. 

 He describes the outcrop as appearing on the contact of the marbleized 

 limestone and the porphyry. The outcrop was 40 to 50 feet wide and 

 was visible for seven or eight hundred feet and might extend much 

 farther. The surface ore sometimes showed copper stains. The sam- 

 ples yielded 60 per cent iron and were low in phosphorus. Dr. Ledoux's 

 descriptions are thoroughly characteristic of the western deposits of iron 

 ore on the contacts of intrusives with limestones. 



Our specimens of the ore are magnetic and indicate magnetite as an 

 abundant, if not the chief, component. Polished slabs reveal an inter- 

 growth with specular hematite. The ore has many cavities and the 

 polished slabs reveal innumerable small ones. In part, at least, the 

 cavities represent associated pyrites, more or less copper-bearing, but 

 now weathered away. Experience elsewhere leads us to anticipate that 

 as the water-level is approached and passed in depth, the unaltered pyrites 

 will be found. The upper portions of the deposit appear to be thor- 

 oughly oxidized. The presence of lime silicates, especially garnet, with 

 the ore has been recorded. The specimens kindly collected for us by Mr. 

 Fowler did not show them, and the senior writer regretfully states that 

 materials have not been available for petrographic details. The exigen- 

 cies of our field-work prevented further collecting. 



It is unfortunate that this great ore body is as yet so remote from iron 

 works and centers of population. The ore must remain as a reserve for 

 the future. L. G. Westgate^** has recently described very similar deposits 

 in the Little Belt Mountains, near Stanford, 57 miles southeast of Great 

 Falls. They are on the contact of intrusive porphyry and the Madison 

 limestone; are chiefly specular hematite, with some magnetite; are low 

 in phosphorus, and in most samples yielded above 60 per cent iron. In 

 the Cordilleran region practically all the bodies of iron ore are now 

 known to be of this character. 



Other economic resources. — Thirty years and more ago interest in 

 prospecting in the Hills was active. Dr. Ledoux speaks of gold-bearing 

 quartzite traversed by him in ascending the southern laccolithic summit 

 of East Butte, and that good colors were found in the gravels of gulches 

 crossing it. This quartzite must have been one of the Jurassic or 

 Cretaceous sandstones or shales locally mineralized by the igneous rocks. 

 He also speaks of observing evidence of galena or gray copper near the 



" L. G. Westgate : Deposits of iron ore near Stanford, Montana. U. S. Geol. Survey 

 Bull. 71 5F, 1920, pp. 85-92. 



