Weed and Pirsson — Highwood Jits. Laccoliths. 17 



one approaches the butte there in an increase in the feldspathic 

 components, and the rock of the columnar cliffs is perceptibly 

 different from that of the lower outcrops. In the syenite of 

 the platy mass on top this difference is, of course, much more 

 marked ; the rock is not so strikingly leucocratic as the syen- 

 ites of Square Butte and the Shonkin Sag laccolith, but is still 

 a syenite. 



Thus the same order holds as in the other laccoliths, a basic 

 outer and lower mass, above a more feldspathic upper, inner 

 portion, and there has been, therefore, the same kind of differ- 

 entiation as in the other laccoliths, only more gradually ex- 

 pressed. 



Laccolithic Character 1 . — From what has been said we infer 

 that Palisade Butte represents the remains of a laccolith for- 

 merly of greater size. We are led to this conclusion by its posi- 

 tion, structures, and associations. 



The three laccoliths form a transitional group ; the Shonkin 

 Sag is the flattest and also the lowest and therefore the one 

 most protected from erosion. Its top, in fact, is just beginning 

 to emerge, and its laccolithic character would not be so evi- 

 dent if it were not for the trenching in it by the former river 

 action which has given such good cross sections. Square 

 Butte stands much higher and has been exposed to much 

 greater denudation ; its cover, save in small areas around the 

 base, has been stripped off and a considerable part of the 

 igneous rock removed. Palisade Butte, standing at the same 

 level as Square Butte, has suffered from the same amount of 

 erosive agencies, but being smaller in size the relative effect 

 has been greater and the cover has entirely disappeared, and a 

 large part of the laccolith, so that around it the floor is exposed 

 and only the central portion of the mass remains. 



TJ. S. Geological Survey, Washington, D. C, and Yale University, 

 Xew Haven. Conn., April, 1901. 



Am* Jour. Sci.— Fourth Series, Vol. XII, No. 67.— July, 1901. 

 2 



