Weed and Pirsson — Highwood Jits. Laccoliths. 11 



Sections of Laccolith. 



Center Outer wall 



feet. feet. 



Leucite basalt porphyry 5 10-15 



Dense shonkinite 5 



Shonkinite 5-6 



Transition rock 3 



Syenite _..___ 25-30 



Transition rock 15 



Shonkinite ._ 60-75 75 



Leucite basalt porphyry ._ 15 15 



Total (approx.) ... 140 100 



From these data we ean now construct a cross section of the 

 laccolith which will have the appearance shown in fig. 9. 



Fig. 9. Cross section of Shonkin Sag laccolith. 



In this figure the cross section is in its natural proportion 

 as regards the relation of the vertical and horizontal scales, 

 and this shows the sheet-like or flattened character of the 

 laccolith. The transition rock and the syenite are included in 

 the uncolored portion of the section ; their vertical thickness 

 is known, but their horizontal extension is from necessity 

 largely conjectural, since we have the distance of its extension 

 from the center in only one direction towards the outer cliff 

 wall side, as revealed in the dissecting gulch. 



Fig. 10. a, Syenite; &, transition rock; c, shonkinite ; d, leucite basalt'por- 

 phyry. Vertical scale six times the horizontal. 



Fig. 10 is simply an enlargement of the preceding diagram 

 in order to exhibit better the relation of the parts. The verti- 

 cal scale is six times that of the horizontal in order to bring 

 the diagram within practical limits. This does not, of course, 

 affect the relative ratios of thickness of the different layers to 

 one another. 



