TRACHYTE BODY. 317 



origin than this formation. The trachyte flows have extended northward from 

 this point, forming the mass of hills in the bend of the ProvQ.,Eiver, between 

 Provo Valley and Kamas Prairie, and extending up on to the flanks of the 

 Wahsatch east of Clayton's Peak, while farther ncrthward they fomi the low, 

 rolling hills between the Weber River and Parley's Park, and doubtless orig- 

 inally covered a very considerable extent of tlie Cretaceous beds exposed 

 along the Weber, being still found on the summits of the ridges to the east 

 of it at Rockport, and forming the walls of the lower canon of Silver Creek 

 above Wanship. 



These trachytes, while presenting considerable variety m texture and 

 appearance, have, nevertheless, a generic resemblance in all the various 

 exposures. They are generally highly crystalline rocks, having a small 

 proportion of groundmass with a large development of sanidin crystals, con- 

 taining remarkably fresh hornblendes and micas, and particularly character- 

 ized by the frequent presence of augite and tridymite. Among the trachytes 

 exposed in the canon of the Provo River, between Kamas Prairie and Provo 

 Valley, is found a rock of exceptionally porphyritic character. It has a red- 

 dish-purple groundmass, in which are small white crystals of sanidin, red- 

 dish-brown earthy hornblendes, with a very considerable development of 

 specular iron. No mica is visible to the naked eye. Through the magni- 

 fying-glass, the groundmass is seen to contain crystals of plagioclase asso- 

 ciated with the sanidin, A still more compact variety in the same locality 

 shows occasional flakes of bronze-colored mica, with scattered crystals of 

 sanidin and black hornblende, imbedded in a reddish, almost homogeneous 

 groundmass. 



About midway in this canon are found beds of blue limestone, having 

 a slight dip to the south and west, exposed for a distance of about half a 

 mile along the edges of the stream, and apparently overlaid by the red sand- 

 stone of the Trias, of which fragments are found in the debris slopes, though 

 the rock was not seen in place. The Triassic sandstones are, however, found 

 cropping to the south of this ridge, on its flanks toward Provo Valley, just 

 north of Heber City, where they have a distinct southwesterly dip of about 

 25°, with a northwest strike. 



On the opposite side of this valley, at the mouth of the canon of 



