Weed and Pirsson — Bearpaw Mountains of Montam^a, 195 



sedimentary strata. This dike appears at intervals as a wall, 

 standing in relief above the slopes beneath the great cliffs east 

 of the stream. On the west it forms a conspicuous reef in the 

 slope immediately above the creek bottom, but was not seen 

 beyond. The dike is 5 feet wide and cuts rusty, massively 

 fracturing hornstones that belong to the contact zone and dip 

 at 15° up stream. The dike crosses the creek at right angles, 

 but trends toward the stock. The western exposure forms a 

 flat-topped wall that is but a few hundred feet long, shown in 

 the accompanying sketch (fig. 3) in which the tilted sediment- 

 arv beds are seen to the left of the dike. 





,','•' i'* •'''c?*''••t;i•■<^"!^^^^'^^^^^ 



Fig. 3. Dike of Tinguaite on Beaver Creek. Bearpaw Mts. 



The dike has a nearly vertical contact, but sends a little 

 stringer out into the shale, as shown in fig. 4. At the con- 

 tact the shales are highly altered for a few inches. 



The dike consists of a bright green porphyritic rock of 

 quite unusual character. It is a pseudo-leueite sodalite tin- 

 guaite. The specimen and section show that this is the rock 

 which has already been described by us "^ from a drift pebble 

 collected on the Missouri River. The specimens and sections 

 are so absolutely identical, and the peculiarities of the type so 



* This Jour., vol. 1, p. 394, 1895. 



