4:24 



D. R. Semmes — Tertiary Intrusives of 



The contact effects, resulting in the baking of the wall 

 rocks for some feet from the contact, have given rise to 

 two belts of resistant rock — the contact zones — between 

 which lies the dike forming a slight depression, the whole 

 resembling somewhat a modern race-course. Toward its 

 western end the width of the dike is 32 feet but it widens 

 to the east and where last seen showed a width of 47 feet. 

 This increase in thickness to the east is decidedly sugges- 

 tive and indicates that both of these dikes are possibly 



Fig. 5. — Photomicrograph of Devil 's Eaeetrack dike. 50 X. Showing 

 diabasic texture. Magnetite and secondary amphiboles in the interstices of 

 the lath-shaped feldspars (P). 



associated with some concealed center of igneous activity 

 farther to the east. The trend of the Devil's Racetrack is 

 exactly the same as that of Railroad Mountain (N 89° E) 

 toward its western exposure. About Sec. 15, T-10-S, 

 R-28-E, the trend changes to N 85° E and continues east- 

 ward in this direction as far as it can be traced. The con- 

 tact effects are confined to zone varying from 1 to 10 or 12 

 feet in width and consist for the most part of a mere bak- 

 ing of the country rock. Numerous prospects have been 



