THE LOMBARD OVERTHRUST AND RELATED FEATURES 289 



porphyry. The granite has a well-developed set of joints which 

 strike northeast and dip 8o° east, and are about parallel with the 

 contact with the Belt Series. 



The age of the granite cannot be definitely determined at this 

 place, but it is probably about the same age as the granitic and 

 syenitic intrusions of the Little Belt Mountains, which are post- 

 Cretaceous and probably early Tertiary in age. 



DIORITE 



Small irregular intrusive masses of diorite and diorite porphyry 

 occur in the vicinity of Dunbar's mine, north of Three Forks. 

 These intrusions cut the white Tertiary limestones which at this 

 locality are considered to be of Lower Oligocene age. The diorite 

 was observed to have nearly vertical contacts with the limestone 

 and to occupy a much smaller area than is indicated on the geologic 

 map of the quadrangle. The diorite porphyry seems to be a local 

 variation in the normal diorite and its distribution can be shown 

 only on a detailed map of the district. 



Specimens of fresh diorite were obtained from the dump at 

 Dunbar's mine. The rock from the main shaft is of medium fine 

 texture and evenly crystalline. It consists of an even mixture of 

 black hornblende and gray feldspar. Under the microscope the 

 rock is seen to consist of greenish-brown to dark-green pleochroic 

 hornblende and labradorite feldspar. Apatite, olivine, and mag- 

 netite occur in small amounts as accessory minerals. Specimens 

 of diorite from a shaft about a half-mile to the south show a small 

 amount of pale-pink orthoclase feldspar scattered through the 

 rock. 



Some of the diorite from a small intrusion which cuts the 

 Cambrian formations a few miles north of the mine is distinctly 

 porphyritic and consists of hornblende phenocrysts in rather 

 slender crystals about a half-inch long in a gray ground-mass of 

 plagioclase feldspar and hornblende. Magnetite and apatite occur 

 in small amounts scattered through the ground-mass and are 

 visible under the microscope. The rock is deeply weathered at the 

 surface and the hornblende is mostly altered to chlorite, and the 

 feldspar is kaolinized. 



