THE BASIN PROVINCE 123 



impart a reddish tinge to the rocks. The abundant mica is mostly muscovite, 

 biotite being present in subordinate quantity. The muscovite is probably of 

 secondary 7 origin, for it is present in much greater quantity than could reasonably 

 be expected, if it were directly derived from the Archean, and in larger leaves than 

 is common among the gneisses observed. 



" In the Mosquito Range two beds of limestone, each about 50 feet in thickness, 

 are found about the middle of the formation. In this district the limestones are 

 more prominent in the upper part, but are very irregularly developed. Six 

 different beds were observed on the south face of Sheep Mountain, but at other 

 points not over two could be detected. They are generally rather thin, but the 

 principal bed in the northwestern part of the district is 60 or 80 feet thick. At 

 the southern boundary of the area this bed can not be detected, and has appar- 

 ently thinned out. The great variability of the many thin beds of limestone in 

 this and the succeeding formation is so remarkable that these have been desig- 

 nated on the map by a special color, which shows approximately the variable 

 extent of calcareous sedimentation in the midst of a great thickness of prevailingly 

 coarse siliceous deposits. The limestone beds are also important in defining 

 horizons, and to them are confined a large and important class of the ore deposits 

 of the district. 



"The limestones in the Weber grits are all typical dolomites, with a small 

 but persistent admixture of carbonates of iron and manganese (1.5 to 5 per cent) 

 and up to 10 per cent of insoluble matter. 



" Maroon formation. This name is here applied to about 1,500 feet of beds 

 which in many respects resemble the Weber grits, but which in the Mosquito 

 Range have a much larger proportion of calcareous and argillaceous beds. This 

 formation in the Tenmile district consists predominantly of coarse gray and red 

 sandstones, in some places passing into conglomerates, with many irregularly 

 developed beds of limestone. As contrasted with corresponding members of the 

 Weber grits, the following distinctions have been noted. The red color of the 

 sandstones, which is more common than in the lower formation, though less pro- 

 nounced than in the beds of the next above, results not from the presence of pink 

 feldspar, as in the Weber grits, but from abundant iron oxide impregnating the 

 cement. Hence in depth, as shown in underground workings, the red color 

 generally gives way to a greenish gray. The strata in which the red color is most 

 pronounced are fine-grained and often somewhat schistose. These sandstones 

 also contain a large proportion of feldspar fragments. In one case it was found 

 that a mixture of carbonates had almost wholly replaced the abundant feldspars, 

 which, nevertheless, were to all outward appearances quite fresh. 



"The argillaceous shales are often black, but seldom contain actual coal; 

 they are much more abundant than would appear from a hasty inspection of the 

 hill slopes, where their outcrops are readily obscured by the debris from the harder 

 rocks. 



"The limestones of the Maroon formation constitute, however, its most 

 characteristic feature, and, independently of color, seem to afford the safest means 

 of distinguishing it from the Weber grits. In outward appearance they are 

 purer and are evidently freer from arenaceous material. They are light bluish 

 gray or drab in color, becoming white on weathered surfaces, in strong contrast 

 with the dirty brown weathered surface of the dolomites. They have also a 

 conchoidal fracture and lithoidal structure, instead of the rough granular fractured 

 surfaces which characterize the latter. Chemical analysis confirms these indica- 



