228 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



topped spur in the middle of the plateau. Small dikes cut the northern 

 edge of the plateau and a small spur from it, and trend toward the north- 

 west at various angles. They can be seen at a distance of 4 or 5 miles 

 traversing the country to the north in the same direction, but of course it 

 is only the largest and most prominent that can be recognized in this way. 



The basalt sheet on top of the plateau is massive and is jointed in 

 large rectangular blocks. The walls of the amphitheater appear to consist 

 of similar massive rock for a depth of a thousand feet, but they are highly 

 indurated breccia. 



The western half of the mesa is cut off by the gulch west of the 

 round-topped spur, and presents a cliff facing eastward. This is intersected 

 by numerous dikes, one of which is 12 feet wide. The cliff passes south 

 into the pinnacled spur west of the gulch. 



A high and narrow ridge extends around the northern side of the head 

 of the twin gulches, and consists of massive rock cut by a few dikes trend- 

 ing north. From it a high spur runs toward the northeast. It is composed 

 of chaotic breccia, which is somewhat indurated and is traversed by a 

 number of dikes trending northeast. 



The portion of Hurricane Mesa east of the twin gulches is less 

 indurated than that immediately west, and contains fewer dikes. Its 

 northern side was not explored, but undoubtedly exhibits some dikes. 

 Across the upper part of the southern slope there is a long straight dike 

 trending south of east; and several others occur lower down the slope, and 

 trend toward the round-topped spur. The southeastern spurs are traversed 

 by 10 dikes, which trend toward the northeast and converge toward a point 

 in the gulch which drains the eastern end of the mesa. Near this point on 

 the long eastern spur the breccia is indurated and a large body of fine- 

 grained crystalline rock is exposed, which is probably connected with the 

 center toward which this group of dikes converge. Near the junction of 

 Closed and Timber creeks a number of dikes were observed, most of which 

 trend toward the northeast. There is a large one trending toward the 

 round-topped spur. Where the two systems of dikes intersect, the dikes 

 from the round-topped spur are found to be the younger, since they cut 

 those trending toward the eastern center. 



These observations have been plotted on the map in such a manner as 

 to show where the dikes have been actually found, the dotted lines being 



