290 WINTHROP P. HAYNES 



There are zones of altered rock along the contacts of the diorite 

 and the Tertiary limestone which are well exposed about Dunbar's 

 mine. In this contact zone are many secondary minerals which 

 include garnets, and several copper-bearing minerals, chiefly 

 chrysocolla, with some malachite and azurite. It is the presence 

 of these minerals which has caused the development of Dunbar's 

 mine. This mine was not in operation during the summers of 191 2 

 and 1913 when the writer visited the region. 



DIABASE 



A rather large intrusion of diabase was observed by the writer 

 in the extreme northern part of the region, about a mile west of 

 Lombard. This somewhat irregular dikelike intrusion follows 

 the plane of the thrust fault across the double horseshoe bend of 

 the Missouri River and varies in width from 100 to 500 feet. The 

 intrusion has produced a noticeable contact effect on the country 

 rocks, particularly on the Cretaceous rocks on the east side, which 

 are indurated near the contact. 



The diabase is deeply weathered near the surface and has a 

 rusty brown color. It forms a very conspicuous massive wall on 

 the north side of the Missouri canyon, northwest of Lombard. The 

 rock shows the ophitic structure well and is composed of augite 

 and labradorite with some olivine, magnetite, and apatite. The 

 age of this intrusion cannot be very definitely placed but it is clearly 

 post-overthrusting, and therefore of Tertiary age. 



SUMMARY 



The contributions of this article may be summarized as follows: 



1. A new geologic map of a portion of the Fort Logan region, 

 and a revised geologic map of a part of the Three Forks region. 



2. The recognition of an extensive overthrust in the north- 

 western part of the region, "the Lombard overthrust." 



3. New facts relative to the stratigraphy of the region mapped, 

 including the identification of a portion of the Belt Series, and the 

 recognition of a disconformity between the Yogo limestone and the 

 Jefferson limestone. 



4. Detailed sections of some of the Paleozoic formations. 



5. The igneous rocks and their manner of occurrence. 



