dl)8 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



METAMORPHIC AND ALTERED EOCKS. 



Metamorphic rocks, such as gneiss, mica schist, clay-slate, and others, are but 

 sparingly distributed over this section, and seem to be mostly confined to the imme- 

 diate proximity to the granites. They occur in the Wahsatch range, the Goshoot 

 Mountains, the Montim range, the Black Mountains near Carson River, &c; but only 

 m the Sierra Nevada they are more considerably developed. The stratified rocks all 

 over the district have, however, undergone great changes by the influence of the 

 igneous eruptions, either directly, by mechanical force and heat, or by chemical agen- 

 cies accidentally connected with the outbursts, such as alkaline waters, &c. They have 

 been tilted, and brecciated, and baked; secretions of siliceous matter have been pro- 

 duced, and agate and jasper formed. In numerous places sandstones have been altered 

 into compact Hint rock; in others, they have assumed a porphyritic appearance, in 

 consequence of a beginning secretion ot crystalline quartz from the siliceous matrix, 

 which has attained n uniform, even texture. I only mention specimens No. 273, from 

 Ivobah \ alley, and No. 288, from Reese's River. 



In slaty rucks such a change cannot be easily traced, because, by being similarly 

 ailected, they at once assume the aspect of truly eruptive rocks; and an appearance 

 ot stratification cannot be regarded as conclusive evidence of the sedimentary origin. 

 It may be the result of the peculiar circumstances under which a fluid mass has cooled, 

 or of successive volcanic effusions. I have observed several instances where igneous 

 rocks f-.nned what appeared to be regular diversified strata, one above the other, re- 

 ( i ,lin1i - :1 r;,lvtul examination to convince me that the rocks were not originally aque- 

 ous sediments, and altered or semifused, but truly eruptive. In other instances the 

 distinctions are less obvious. Igneous rocks in such thin strata, like those in veins, 

 generally exhibit a different appearance from those in larger bodies, because they have 

 cooled quicker in contact with cold surfaces, whereby the free play of the molecular 

 attraction and the separation of the constituent minerals is impaired or even forced 

 into a different direction. Instances of that kind will be mentioned in the description 

 ot the single ranges of mountains. 



In the deserts east of Carson Lake I have observed a mountain of white dolomite, 

 n!'!uman!'iif!irut!h!'n d ^'^ ""V"™" ")»««<»*> "'Im-h still forms part of the 



.1. .7 , -"■«"« "ww m me lormanons m Western Utah, not even along- 



the traveled routes. On the geological map of Professor Hall, in the Report on the 

 Mexican Boundary Survey, a large portion of it is colored as metamorphic, and the 

 remainder is left blank. In Captain Beckwith's report merely "limestones" are men- 

 tioned occasionally, but their age had not been determined. That Upper Carbonifer- 

 ous limestones occurred near Salt Lake, was the only fact satisfactorily establld,,,! 



iTLZlZTT U ; th V iel( V md 0U V° lleCti0n8 ' mUCh ^^[information 

 as been de m ed They have largely contributed to our knowledge of the extent and 

 development ot the geological formations, and have also proved the existence of some 



