GEOLOGICAL SURVEY OF THE TEEEITOEIES. 21 



The story of the changes which have occurred in the geological 

 history of this great interior basin can, no doubt, be traced by uniting 

 link to link the internal evidence of the rocky strata from the earliest 

 period to the present time, and this work belongs to the province of the 

 geologist. To contribute something toward the task of reconstructing the 

 physical geography of this country in past geological times is my principal 

 object in writing out the geological features of our route in so great detail. 

 There is so much similarity in the general structure of the country that I 

 might express the more prominent points in few words, but this would fail 

 to give definiteness to the work. At the risk of repetition, I shall present 

 the principal facts observed during each day's travel in the order in which 

 they were obtained. 



As we proceed northward the hills on the east side of the valley 

 become more irregular and broken. Massive beds of the limestone 

 can be seen as far as the eye can reach, capping the summits, and 

 inclining from the valley eastward at various angles, but the lower 

 hills in front are much rounded and covered with grass, showing the 

 softer character of the underlying rocks. Clays, sands, and quartzites of 

 various textures prevail. On the west side the nucleus of the mountains 

 is undoubtedly the same ; but high up on the summits, as well as on the 

 sides, are found the yellowish and whitish marls and sandstones of the 

 later Tertiary or lake deposits, filling up, to some extent, the inequalities 

 of the surface, and sometimes inclining, at a moderate angle, in the same 

 direction with the older rocks beneath ; with the latter, however, the 

 former do not conform. This range of mountains, which continues unin- 

 terruptedly nearly to Marsh Yalley, on the west side of Eound Valley, 

 rises 1,000 to 1,200 feet above the bed of Bear Eiver. The summits are 

 covered with bowlders, mostly quartzites. The outline of this range is 

 formed of an irregular series of cones, with a general dip to the east. 

 The inclination is quite irregular, sometimes 10°, then 60° or 70°. There 

 is so much material of a soft nature that yields readily to atmospheric 

 influences that the underlying harder strata are much concealed, so that I 

 found it impossible to obtain a continuous section. The mountains on 

 the west side from the crossing of Bear Eiver to Bridgeport present a 

 very abrnpt front toward the valley. Originally the quartzites, clays, 

 and limestones were elevated so as to correspond with the ridges or 

 hills on the east side, inclining in the same direction, but the outburst 

 of igneous rocks has produced some changes in position since the elevation 

 of the older rocks. The igneous rocks haye the peculiar somber hue and 

 abruptness of basalts, and, in this case, they would appear to have been 

 thrown up under great pressure, so that they are exceedingly compact 

 and massive on the surface, and even where the little streams have 

 flowed down the sides, forming deep caiions, the same close texture is 

 shown. Wherever the sedimentary beds come in contact with these 

 basalts, they are changed more or less. The clays are changed into 

 bluish slates, the quartzites are more crystalline, and the limestones are 

 more or less metamorphic, and exhibit a darker hue. They are also 

 full of cavities, lined with quartz crystals, or calcite, and seams of 

 quartz. In this range of hills or mountains, near Bridgeport, silver 

 mines have been found. One lode has been discovered that yielded ore 

 which is said to assay $75 per ton. It is not probable that this will 

 ever be a successful mining district, and however rich the ore may be 

 in localities, it will doubtless be found only in pockets, and not in 

 regular lodes. The area is limited, and whatever ore there may be, it 

 has probably been segregated in fissures or cavities by the action of the 

 basalts on the contiguous quartzites. Originally the quartzites and lime- 



