64 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Siluriiui), are clearly shown, ami yet, at a distance of thirty miles to the 

 northward, this entire grou[) sceius to be wanting:. 



In the rei)ort of the survey of last season I described the country 

 about Virginia City in brief terms, and but little more can be added 

 from the examinations of the present season. Alder Gulch is well known 

 to the mining world as one of the richest placers in the West. It is 

 estimated that about thirty millions of gold have been taken out of it. 

 Tlie bowlder-drift is a marked feature in the gulch. Upon the sides of 

 the hills i)atches of black basalt may be seen in considerable numbers, 

 iudi(;ating the very latest period of eruption. The older trachytes also 

 occur on both sides of the gulch, which have been erupted at diflerent 

 periodsand haveovertlowed the gneissic rocks. None of theigueons rocks 

 observed by me, however, were old in a geological sense; none that date 

 back further than the Pliocene epoch. A more careful scrutiny of the 

 rocks might have resulted in detecting igneous rocks of different geolog- 

 ical ages, but very few seem to date back beyond the Pliocene. It is 

 not uncommon to tind the basalts interstratitied with Cretaceous and 

 Tertiary beds, but I do not think it follows that the eruption took place 

 during these periods. In the high bluff opposite the Mammoth Hot 

 Springs, on Gardiner's River, thick beds of basalt are exposed in the 

 upper portion of the blufl of irregular extent. On the summit is a very 

 thick bed lying across the upturned edges of the strata, and the line of 

 contact is quite red, showing the influence of the melted lava on the 

 sedimentary strata immediately beneath. I am disposed to believe these 

 short intercalated beds of basalt may have been of very modern origin. 



At the head of Alder Gulch there is a high wall of limestone of which 

 Old Baldy is the highest point. The aggregate dip seems to be about 

 southeast 30° to 45°. The exposure is a fine one, with 1,200 to 1,500 

 feet of vertical strata. The lower portions of the sedimentary beds are 

 quartzose, sometimes pebbly, very hard, compact, gradually passing up 

 into limestones, which are also very hard and splintery, destitute of fos- 

 sils. But in the upper portion of both sides of Old Baldy the char- 

 acteristic Carboniferous fossils are very abundant. Thick beds of lime- 

 stones are a simple aggregate of well-preserved fossils, and among these 

 some very interesting crinoids. For a list of the fossils from this local- 

 ity the reader is referred to the catalogue of Mr. Meek. The high ridge 

 or mountain-range of limestones, of which Old Baldy forms a part, trends 

 off to the southwest, across the head of the Passamari, Black Tail Deer, 

 and Red Rock Creeks, to an unknown distance. These huge ridges, 

 which extend in such long lines across the Northwest, are undoubtedly 

 portions of grand anticliuals. It will prove a great source of pleasure 

 at some future time to trace these great axes of elevation across the 

 country and aggregate them into a symmetrical grou[). Until this is 

 done it will hardly be possible for us to comprehend the correct topo- 

 graphy or geology of this region. 



We will return to the Madison Valley. That the period of the 

 eruption of the igneous rocks began before the mountain-ranges had 

 reached their present height and form is shown by the position 

 of some of the ridges of trachyte that extend down the sides of the 

 mountains into the valley on the west side of the Madison. The ridges 

 of trachyte have been so eroded that the bluff-sides present a stratified 

 appearance and the inclination is 5° to 10°, the beds passing from near 

 the summit of the divide down beneath the superficial or lake deposits of 

 the basin. As we descend the IMadison the range of mouutaiiis on the 

 west side is more purely metamori)hic, few of the igneous rocks being 

 observed, and no sedimentary at all. This continues nearly to the Jeff- 



