262 



G. R. MANSFIELD IGNEOUS GEOLOGY OF IDAHO 



(his Station XVII) as the cause of the upheaval of the mountain. He 

 says : 



"Although the deposits in the immediate southwest slope of Station XVII 

 are somewhat obscure, and withal so altered by metamorphic action as to 

 render their examination difhcult, yet their more favorable exposure in the 

 opposite hillside to the west affords satisfactory data for the determination of 

 the relations of the sedimentaries to the volcanic phenomena with which they 

 are here associated. The igneous mass protruding in the crest of the ridge 

 seems to have been forced up nearly in a vertical direction, carrying the sedi- 

 mentary beds up with it instead of fracturing them at once, so that at the 

 extremities of the upthrust they were not rent apart. But at the point of 

 greatest tension they were partially fractured, the igneous matter following 

 the crevice thus produced, as a wedge-shaped mass, which subsequent erosion 

 has bared, and thus revealed the origin of the little anticlinal fold, of which 

 it forms as well the nucleus." 



Saint John's geologic structure section at Sugarloaf Mountain is repro- 

 duced in figure 2. More detailed study of the Sugarloaf district has 

 shown that the Homer limestone into which the andesite is intruded is 



soo-,H. 19 Basalt and scoriaceous/^r^s. /-a, //-/8, Laramie beds. lOHomblendic Trachyte 

 Scale ol J iMi/es Base SOOofL aboife sea-le.rel 



Figure 2. — Soint John's structure Section through Sugarloaf Mountain 



folded into an inverted, fan-shaped sj^nclinorium, of which the anticline 

 at Sugarloaf Mountain is one of the minor folds. The intrusive body is 

 a thickened sill or incipient laccolith that, in the northwestern extension 

 of the mountain, arches with the strata; but beneath the summit the 

 southwest limb of the anticline is so eroded that the sill is exposed as a 

 southwest-facing cliff. These features are shown in the accompanying 

 map and geologic structure section, figure 3. 



The andesite shows no shearing and the minerals in thin sections shoAV 

 no strain. These facts indicate that the rock was not folded after its 

 intrusion, but that it either followed structure lines already established 

 or itself participated in the deformation of the associated strata. The 

 latter view seems more probable, for Lindgren/" after a study of the 

 Cordilleras, says : 



i» Waldemar Lindgren : Op. cit., pp. 282-284. 



