16 Weed and Pirsson — Flighwood Mts. Laccoliths. 



though often greater, divided by regular crossjoints. They 

 extend the whole height of the exposures in which they occur 

 and thus attain a length of 100 to 150 feet in places, but it is 

 evident that originally they must have extended through the 

 whole of this portion of the igneous mass, as shown in the 

 Shonkin Sag laccolith, and therefore may have been several 

 hundred feet in length. It is, of course, by the breaking 

 away of these columns that the vertical cliffs have been 

 formed. At a distance on the plain the butte appears faced 

 by a series of colossal pillars or palisades, and from this appear- 

 ance it has received its name. It strongly recalls " Mateo 

 Tepee " or the " Bear Lodge Butte " on the west side of the 

 Black Hills. 



The rock composing the columnar portion of the butte and 

 the outcrops in the talus is- of shonkinite of a somewhat firmer 

 texture and perhaps more feldspathic character than that of 

 Square Butte. On all exposed surfaces it weathers to a very 

 dark color, giving the cliffs a sombie and gloomy character. 



In several places the line of cliffs is cut by deep and narrow 

 gulches descending through them, up which one can easily 

 clamber and attain the grassy slopes and shoulders above them, 

 and thus by continuing reach the summit. These grassy slopes 

 are varied here and there by groves and clumps of small pines. 



The butte is crowned at the top by a large mass of a light- 

 colored rock of a platy, laminated character. This is an augite 

 syenite much like that already described. It weathers with 

 a much lighter color than the shonkinite on account of the 

 preponderance of the feldspar over the augite, and the con- 

 trast between the two, while not so pronounced as at Square 

 Butte, is still a striking one. 



The mass of syenite, which has a considerable thickness, is 

 roughly wedge-shaped in form, with a cliff of some height 

 toward the south and a slope toward the north. On the for- 

 mer side, the rock with its platy, horizontal parting is clearly 

 seen to be resting on the massive, columnar shonkinite, but 

 towards the north the relations are less clear because in this 

 direction it slopes down in talus masses and in fact, on this 

 side the butte, to a great degree, loses its precipitous character 

 and slopes sharply down toward the plain, with the broken 

 down rock masses intermingling. 



Differentiation. — In the characteristics of the rock compos- 

 ing it, there is to be seen the same differentiation as that found 

 in Square Butte and the Shonkin Sag laccolith, only not so 

 sharply expressed. In the low outcrops of rock in places 

 which rise through the talus slopes and are furthest from the 

 butte, the shonkinite composing them is very rich in augite 

 and therefore dark and basic, like that of Square Butte. As 





