FARMINGTON AND WEBER REGION. 375 



with recent soil, but outcropping sufficiently to indicate a thickness of about 

 800 feet of beds entirely conformable with the overlying limestones, which, 

 from its position, is doubtless the Ogden Devonian. Under that, again, 

 appears obscurely a mass of shaly gray limestone, earthy in places, of 

 irregular dip, but occupying the position of the Ute limestone. Traced 

 eastward on their strikes, all this series of Palaeozoic rocks, within a dis- 

 tance of a mile, run under the rudely-bedded Tertiaries, which, although 

 in general horizontal, show here a dip of 8°, and even 10° and 11°. 



These Tertiaries consist chiefly of conglomerates made up of pebbles 

 of limestone, quartzite, and sandstone, among which the former is con- 

 spicuous, and a varying cement of a brick color, partly calcareous and 

 partly siliceous. Unlike any others in the vicinity, they are more or less 

 riven by deep cracks, and contain zones where the whole material is so 

 confusedly crushed as almost to have lost the appearance of bedding. 



In the canon of City Creek, about two miles above its entrance, the 

 bottom and the hills on either side for half a mile are occupied by a mass 

 of trachyte ; contemporanous erosion has so obscured the relations of this 

 body with the Tertiary which surrounds it on all sides, that it.was impos- 

 sible to decide which is the later. The prevailing hue of the rock is a 

 light reddish-gray, in some instances passing into an almost chocolate color, 

 and in others, a true pure gray. It is very remarkable for the richness of 

 its mineral ingredients. The feldspars are both sanidin and plagioclase, 

 with a decided predominance of the latter; many brownish-black and 

 bronzy-brown biotites occur, and black, well-preserved hornblendes up to 

 a quarter of an inch in length. Under the microscope, the hornblendes are 

 extremely interesting as containing fluid-inclusions with moving bubbles, 

 and within an exterior coating of dark hornblende are nuclei of pale color- 

 less hornblende. The rock is also rich in microscopic tridymite, almost 

 equalling in its abundance the rock of Cerro de San Cristobal in Mexico. 

 It contains no quartz, but numerous apatites; augites, usually visible only 

 under the microscope, but rarely visible to the naked eye. In many places, 

 the biotites have sufiered more or less decomposition, and are of a brilliant 

 garnet color. The rock has decided affinities with the augite-andesites, 



