126 BULLETIN OF THE ESSEX INSTITUTE. 



tains. Nearer, the beds change gradually to a north dip 

 3° to possibly 5° ; and this very gentle dip continues to 

 within about two hundred feet of the fault, when the beds 

 are suddenly flexed up to a nearly vertical northeast dip. 

 At the fault, which is quite clearly exposed in the 

 bottom of the gulch on the east side of the mesa, the 

 vertical or overturned Fountain beds are in direct con- 

 tact with a large dike of sandstone. The north edge of 

 the dike shows a trend N. W. -S. E. and a southwest 

 hade of at least 30° and possibly 45°. The dike is mainly 

 fine white sandstone, but abundantly mottled with red. 

 It is exposed almost continuously in the bottom of the 

 gulch for a breadth of about seventy-five feet. On the 

 west side of the gulch the upper or south contact may be 

 easily traced, showing the same southwest hade as the 

 lower contact. The dike is here, however, much branched, 

 inclosing large masses of granite and penetrating the 

 granite in numerous sharply defined dikelets one-fourth 

 inch to one foot thick. In some parts of the sandstone 

 numerous small angular fragments of granite are inclosed. 

 About one hundred yards above this sandstone dike is 

 another with the same southwest hade, and a surface 

 breadth of twenty to thirty feet. These two sandstone 

 dikes cross the ridge or spur on the east side of this little 

 gulch to the next gulch beyond, in which the fault is not 

 clearly exposed ; but the Fountain beds are seen within 

 forty feet of the north dike dipping north about 10°. 

 The north or fault dike is here nearly if not quite one 

 hundred and fifty feet wide, and seems to be nearly ver- 

 tical. The sandstone is light gray blotched with red, as 

 usual, with many highly polished and striated slicken- 

 sides or shear planes. There are also some indica- 

 tions of faulting along the south side of this dike. Two 

 to three hundred feet south of this dike is the other, 



