404 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 



nacles, just as rocks condition the formation of ice-tables and pinnacles 

 on glaciers (see figure 1, plate 26). 



In many other cases the white S3"enite bowlders are to be seen lying 

 at the foot of the columns from which they have fallen. 



We may remark here that in all our experience we have never seen a 

 more weird and curious labyrinth of pillared rocks than this which sur- 

 rounds the lower base of Square butte. In singular scenery it equals, if 

 it does not surpass, the famous Hoodoo country, on the northeast border 

 of the Yellowstone National Park, in northwest Wyoming. 



UPPER ZONE OF WHITE ROCK. 



Ascending through the zone of hoodoos which measures perhaps a 

 mile along the slope, they are found to diminish in size and a horizon is 

 reached where the character of the rock changes abruptly from the dark, 

 nearly black augitic phase to the white syenite described by Lindgren. 

 In many places the hoodoos continue, but noAV they are made of the 

 white rock. They are smaller in size, but possess the same remarkable 

 disk like, platy structure, and the disks are perfectly parallel to those of 

 the black variety below. The white hoodoos are rarely pointed, and are 

 more apt to be flat topped and of the character seen in figure 2, plate 26. 



The transition line between the two rock varieties is extremely abrupt, 

 but it is not of the nature of a contact. The rock continues of even grain 

 throughout, but in the space of a few inches or a foot or so the black 

 augite begins to diminish and finally disappears, the rock assumes a 

 more feldspathic character, hornblende occurs, and it rapidly passes into 

 the syenite described by Lindgren and which constitutes the main inner 

 mass of the mountain. There is thus a narrow mottled zone between 

 the black and the white rock. The hornblende is present in so small 

 an amount in the syenite that the rock, especially when seen in full sun- 

 light, has the whiteness of marble. 



The monoliths which lie near the transition zone are sometimes seen 

 to be of black disks resting in place on a pediment of the white rock be- 

 low ; sometimes the transition band passes through them and they are 

 of black disks resting on white ones, or it passes through the disks at a 

 nearly vertical angle so that one part of each disk is white and the other 

 black ; at other times a white hoodoo is but a few feet distant and above 

 a black one, both resting in place on a continuous exposure of the white 

 rock. These facts are illustrated in figure 2, plate 26, which shows a 

 black hoodoo resting on white syenite, with a white hoodoo above and 

 to the left. 



The facts just presented are to be carefully noted, because they show 

 beyond the possibility of a doubt that however much the two varieties 

 of rock may differ and however abrupt may be the change from one into 



