Weed and Pi?*sson — Highwood Mis. Laccoliths. 



Always ascending, the shonkinite is found to have a thick- 

 ness of about seventy-five feet, when it suddenly passes into a 

 rock of quite different character. So far as the exposures show 

 there does not appear to be any contact between the two, but 

 the shonkinite quite suddenly, in a little distance, passes into 

 the new type. This is a much lighter colored rock of a coarser 

 grain, so coarse, in fact, as to have quite a pegmatitic appear- 

 ance. It is composed of large augites, many of them one to 

 two inches long, often radiating from a common center in such 

 a manner as to form stars, and equally long but slender foils of 

 biotite, filled in between with white feldspars or feldspathic 

 material. This rock was always found in so weathered and 

 crumbled a condition that no good fresh specimens of it could 

 be obtained. In thickness, this layer or zone is about fifteen 

 feet and it passes above into a white syenite of medium grain, 

 specked with augite crystals. This has a rather thin, horizon- 

 tal, platy parting, by which it splits and weathers into piles of 

 plates. Seen in the bright light of a clear, sunny day, the 

 contrast with the dark shonkinite makes the one seem white, 

 the other black, especially when they are seen in large masses. 

 In hand specimens, the contrast, though very strong, is not so 

 pronounced. 



This syenite resists weathering 

 much better than the crumbly, tran- 

 sition rock mentioned above and 

 has, therefore, conditioned the for- 

 mation of curious and fantastic rock 

 piles along the line of the outcrop. 

 They commonly take the form of a 

 mushroom or stool in which the 

 outspreading top is formed of the 

 syenite, while the stem is composed 

 of the transition rock. They are 

 illustrated in fig. 5 from a field 

 sketch. These are often fifteen feet 

 or so in height and their mode of formation is much like that 

 of rock tables standing on an ice foot on the surface of a 

 glacier or the breccia hoodoos of the Yellowstone Park. 

 They are somewhat different and smaller than the monoliths 

 of the hoodoo zone around the base of Square Butte, which 

 have been formed in a somewhat similar way. Seen in rows 

 along the hillslopes at the outcrop of the transition rock, they 

 present a weird and curious effect, suggesting crude architec- 

 tural efforts of some primitive race. In other places the 

 transition rock w r eathers back along the outcrop into shallow 



the roof of syenite, the entrance 



Fig. 5. Erosion 

 interior of laccolith, 



monoliths in 





caves, the floor of shonkinite, 



