Weed and Pirsson — Ilighwood Mis. Laccoliths. 15 



Third. There is not the slightest evidence of melting and 

 absorption of the beds in place, in fact, as we have previously 

 described, the amount of contact metamorphism is extremely 

 limited. Moreover, an inspection of the cliff wall fronting the 

 Shonkin Sag, shows that the beds on top of the laccolith are 

 the same as those which elsewhere rest on its basal beds : they 

 have simply been lifted by the intrusion of igneous material 

 and there is nothing lacking in the sedimentary series, a fact 

 which at once disproves the idea of melting and absorption. 



We may thus with the utmost confidence dismiss these two 

 hypotheses, that the arrangement of the rock types is due 

 either to successive injections or to the absorption of sedimen- 

 tary material by a molten fluid and return to our original 

 proposition, that it has been produced by processes which have 

 taken place within the laccolith itself after it had been injected 

 as a homogenous body of magma. 



What these processes may have been will be discussed later 

 in a work on the petrology of the region, but in concluding 

 this description of the geology of the Shonkin Sag lac- 

 colith we desire to again remark, that we believe it furnishes 

 one of the best and most conclusive examples of the differen- 

 tiation of igneous rocks which has yet been described, and we 

 regret that it is not in a more accessible locality for study by 

 other geologists. 



Palisade Butte. 



By reference to the map, fig. 1, accompanying this paper, it 

 will be seen that Palisade Butte is situated somewhat less than 

 a couple of miles west of Square Butte. It stands isolated 

 upon the open, slightly rolling plains country, and though 

 somewhat dwarfed by the nearness of its greater companion to 

 the east, it forms, like Square Butte, a prominent landmark 

 for long distances across the open prairies. It rises some eight 

 hundred feet above the plain and its outline seen against the 

 sky resembles the weathered stump of some gigantic colossal 

 tree. 



On closer inspection its salient features are easily seen. On 

 all sides a long talus slope in great part covered with soil and 

 grass but broken here and there by low outcrops and masses of 

 rock leads up with increasing gradient from the plain to the 

 bare, tall cliffs of naked, massive rock which compose the main 

 mass of the butte. Above these cliffs there are again steep 

 slopes interrupted anew by cliffs. 



These great walls are remarkable for the very regular and 

 beautiful columnar structure of the rock, the hexagonal col- 

 umns being on an average about eighteen inches in diameter 



