no GEOLOGY OF THE EUREKA DISTRICT. 



however, only a partial explanation, as it is difficult to understand why the 

 stream should not continue on its course, cutting its way through the low 

 rhyolite barrier, ratlier than turn to the east and follow the present coui'se, 

 which it finally took across the uplifted sedimentary beds. There seems 

 no doubt that, before the rhyolite eruption, the stream bed followed the 

 canyon and emptied directly into Fish Creek Valley. 



Of the shale formation, little need be said in addition to the descrip- 

 tions already given of the beds. They show great uniformity of deposi- 

 tion and physical character, monotonous in outline and color, and, so far as 

 recognized, carry no organic remains. The sandy, limy transition strata 

 into the Hamburg limestone generally offers better lines of stratification 

 than either the shales below or the limestones above, and the dip and strike 

 may be determined at a number of points along the base of the overlying 

 horizon. 



Hamburg Ridge.— Aloug the east side of Secret Canyon the Hamburg 

 limestone and shale and the Pogonip limestone horizons form a single ridge, 

 which, although of less elevation and of less rugged aspect, is singularly 

 like Prospect Ridge in its salient topographical featui-es. With the excep- 

 tion of the summit of Roundtop, all the more elevated portions are found 

 in the Hamburg limestone. Although evidences of bedding are for the 

 most part obliterated in the Hamburg limestone, they are by no means so 

 exceptional as to leave any doubt that the ridge dips easterly with 

 great uniformity. Occasional beds are found with a dip and strike not in 

 accordance with this general structure, but in such instances they can be 

 shown to be the results of local disturbance produced by the action of in- 

 trusive rhyolites. In studying the district, care has been taken to discrim- 

 inate between such local disturbances, which may be very considerable 

 within limited areas, and the structm'e due to the primary upheaval and the 

 blocking out of the great mountain masses. At the southern end of the 

 ridge the Hamburg limestone has been a good deal broken up under the 

 influence of the rhyolites of Gray Fox and the numerous small dikes 

 of the same intrusive rock. Here the beds are seen standing- nearly 

 vertical, sometimes inclined westerly, and again resuming the normal 

 dip to the east. The limestone beds thi-oughout are highly siliceous. 



