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SCIENCE. 



[N. S. Vol. IX. No. 221. 



dous proportions, two and three miles in 

 width and several thousand feet in height, 

 to narrow dikes and seams traceable along 

 the canyon walls for only a few feet and 

 often discoanected at the surface from 

 any other body. A short distance north 

 of Needle Mountain, but on the opposite 

 side of the canyon, another great stock 

 riges precipitously above the stream bed, 

 and it is clearly evident that its rela- 

 tion to the breccias are in every way 

 similar to those observed at Needle Moun- 

 tain. Between these two massive bodies 

 smaller outcrops of diorite and diorite- 

 porphyry are exposed in lateral ravines on 

 the mountain sides, and the network of 

 dikes trending in every direction points 

 conclusively to the fact that these intrusive 

 bodies belong to one and the same stock. 

 Dislocated and indurated bodies of breccia 

 are found upon the mountain spurs, but 

 the overlying capping of breccia peacefully 

 crowns it all. 



From this point northward, following 

 along the line of the powerful intrusions, 

 each dissecting canyon, where it cuts the in- 

 trusive masses, lays bare numerous expo- 

 sures of crystalline rocks which have forced 

 their way upward into the breccias, and, 

 following lines of least resistance, have 

 spread out in all directions with a marvel- 

 ous complexity of form and outline. Some 

 of the stocks penetrating the breccia have 

 attained elevations slightly above the pres- 

 ent level of the plateau, but most of them 

 failed to reach so high a position. Wherever 

 they have reached the top of the plateau 

 their tendency is to spread out in sheets, 

 which now form the exposed surface of 

 spurs and ridges. Many of these inter- 

 bedded sheets are directly connected with 

 some of the larger stocks, but others show 

 no such relationship at the surface and 

 stand out quite independently of them. Oc- 

 casionally the sheets bulge up with irreg- 

 ular outline; others are dome-shaped, de- 



veloping laccolithic form. Vertical dikes 

 cutting the interbedded bi-eccias pass into 

 sheets, and later again assume the condi- 

 tions of normal vertical dikes. The vari- 

 able character of the breccia, sometimes 

 compact and uniform and at others made 

 up of an incoherent mass of silts and ash, 

 tends to constant change in the upward 

 movement of the molten magma. 



The gorges of both Cabin Creek and Can- 

 yon Creek expose similar rocks, with ac- 

 companying phenomena of strain and rup- 

 ture. Ishawooa Canyon, one of the most 

 rugged of these incisive trenches, presents 

 varied modifications of eruptive energy, a 

 bold stock. Clouds Home, piercing the brec- 

 cias with an irregular outline from the 

 bottom of the canyon to the top of the 

 plateau. One of the finest examples of a 

 massive interbedded sheet extends for a 

 mile or more along the canyon wall. Sim- 

 ilar phenomena present themselves in 

 Wapiti Canyon, where four tributary 

 streams, uniting to make the river, have 

 cut down in the intrusive masses in a most 

 instructive manner. Near the sources of 

 Eagle Creek diorite and andesite-porphyry 

 are again laid bare, and thenge, trending 

 across the crest of the range, extend as far 

 as Sylvan Pass, where coarsely crystalline 

 diorite and diorite-porphyries come to the 

 surface for the last time in an exposure 

 nearly a mile in length. Beyond this point 

 eruptive energy gradually dies out, and is 

 only shown by the presence of a few power- 

 ful dikes noticeable for their uniformity and 

 persistency. 



A distinctive feature along this entire 

 line of intrusive rock is the belt of indurated 

 breccia which accompanies it. Near the 

 larger stocks the alteration of the breccia 

 is especially noticeable, and not infre- 

 quently it is difficult to discriminate be- 

 tween the stock masses and the metamor- 

 phosed material. The mode of weathering 

 is so unlike that of the ordinary breccia, 



