INDIAN CREEK LACCOLITH. 15 



well-known jointing' of igneous rock. The limestone and shale inclosed 

 within the body of the laccolith, also show almost no evidence of meta- 

 morphism other than a lightening of their color in immediate contact with 

 the porphyry. The inclosed stratum of limestone is 20 to 40 feet thick, 

 and rather persistent. It is nearly horizontal, or arches gradually with the 

 curve of the laccolith dome. Several masses of red and green shale, tilted 

 at high angles, were seen in the igneous rock. These are probably blocks 

 of the heavier shale belt forming the upper part of the Flathead formation, 

 and which is the horizon in which the laccolith appears to have been 

 intruded. There are also small fragments of limestone and gneiss included 

 in the porphyry, caught up in its passage through the lower rocks which 

 were ruptured at the time of its intrusion. 



The laccolith sheet thins out eastward under Antler Peak, disappearing 

 near the base of the eastern slope, where the upper and lower limestone 

 strata meet and form the whole of the northeast spur of the mountain, 

 dipping at the low angle of about 5° NE. The apparently gradually 

 increasing dip of the overlying limestones as they arch over the laccolith is 

 found on investigation to be irregular, the dips varying along the cliffs 

 forming the bare southern exposure, increasing from 5° to 10°, and farther 

 west to 20°, then becoming nearly horizontal just before reaching the 

 depression on the ridge. There is, however, a northerly element of the 

 dip which is not noticeable on the southern exposure. Where the dip 

 changes noticeably, the limestone is shattered by innumerable small, 

 vertical faults, close together; it thus behaved as a brittle, not as a plastic, 

 mass at the time of the laccolithic intrusion. The overlying limestone 

 embraces the upper part of the Cambrian formations, including the massive 

 mottled limestone which is the base of the Gallatin limestones, together 

 with the barren strata that represent the Silurian and Devonian, and about 

 400 feet of the Carboniferous, which forms the summit of Antler Peak. 



The laccolith is about 1,200 feet thick at the middle, where it forms a 

 high point projecting into the valley of Indian Creek. Here the limestone 

 capping has been removed by erosion, leaving the slope of the ridge to 

 indicate about the slope of the old plane of contact. The triangular peak 

 northwest of this point shows the overlying limestones dipping at 2° to 3° 

 NW. and extending down the long divide to Bighorn Pass, where they 

 dip over the porphyry for a short distance at 10° and also at 25° NW., 



