L.\C(i)i.rriis i.\ sor'ri/i-:.\sTi-:Rx Colorado 825 



EXPLANATION OF FIGURES. 



All of the figures except the landscape view have the same scale. Each 

 of the three maps represents the same area. To aid in their comparison 

 vertical and horizontal lines are drawn at intervals corresponding to one 

 mile. 



Fig. I. Map of Twin Butte and vicinity, showing the topographic relief 

 by contour lines at everv 25 feet. 



Fk;. 2. Geologic map of Twin Butte and vicinity, showing the distribu- 

 tion of the surface formations described in the text. 



Fk;. 3. Landscape. In the foreground the plain of Neocene alluvium. 

 At the right. Twin Butte. At center and left, the crest of the laccolitic 

 dome, dissected by drainage flowing in the direction from the observer and 

 descending rapidly to Two Butte Creek. The rocks capping these low hills 

 are shales hardened by reaction from the laccolite beneath. 



Fig. 4. Ideal cross-section of the igneous intrusion, from east to west. 

 The profile of this is constructed from the contours of deformation (Fig. 5) 

 on the assumption that the intrusives constitute a continuous mass dividing 

 the sedimentaries at a single horizon. It is known that this assumption is 

 not strictly true, but the meager data at command do not suggest an improve- 

 ment. The diagram shows for each point the total uplift of the cover and 

 therefore the total thickness of intrusive rock. 



Fig. 5. Deformation map of Twin Butte and vicinity, showing, by means 

 of contours at intervals of 100 feet, the form which would appear if the 

 eroded parts of formation 3 (Fig. 2) were restored, and all overlying forma- 

 tions were removed. Where the positions of the contours are controlled 

 by the geologic data the lines are full. The broken lines are interpolated. 

 The other lines of the figure mark the position and trend of dikes. 



