THE BUILDING OF THE TUSHAE. 185 



ries are seen lapping around both the northern and southern extremities 

 of the range, and it is probable that they are concealed not far from its 

 eastern base. 



Such was the relation of the area to its surroundings when the earliest 

 eruptions (so far as they have been observed) took place. They broke 

 forth at first along the course of the present eastern front, a little east of 

 the main divide as it now stands, and along a line nearly 30 miles in length, 

 having a general trend north and south. They were not continuous along 

 this line, but were massed in at least three places: one near the northern end 

 of the Tushar, one (and this the principal one) near the central part of the 

 front, and the other near the southern end, but a few miles southeast of it. 

 The location of this latter center of eruption cannot be fixed at present 

 with exactitude, and may have been more remote than I was at first led to 

 suppose. The interval between the southern and middle sources is greater 

 than that between the middle and northern, and it is not certain that this 

 second or northern interval was well marked, though the southern interval is 

 very distinctly so. What other vents existed, or even whether any others 

 existed at all, it is not now possible to determine, on account of subsequent 

 accumulations which have buried the surrounding country. This period 

 of eruptive activity was certainly a long one ; for between the outbreaks 

 erosion went on, leaving traces of its action in the eroded surfaces of its 

 sheets and in the many small local conglomerates formed out of their decay. 

 But the accumulation by successive outpours was far more rapid than the 

 waste, until there came a long period during which these vents were sealed 

 up and degradation proceeded. At the commencement of this period of 

 repose the eruptive masses must have been piled up to a great altitude and 

 covered an extensive area, for the conglomerates which were formed by 

 their dilapidation are of immense extent and thickness and sufficient in 

 mass to build a goodly range of mountains. The southern interval was 

 almost wholly filled up by the fragments washed into it and stratified, and 

 the conglomerate thus formed stretches far to the southwest, always main- 

 taining a great thickness. At least 2,000 feet of it occupy the southern 

 interval, and it is still many hundreds of feet thick 8 or 10 miles away. 



In many respects the relations of the eruptive masses to the country 



