NEVADA LIMESTONI-:. 65 



Nevada limestone, the physical features of sedimentation are sufficiently 

 characteristic to con-elate the strata when comparing a large number of 

 sections across several thousand feet, although the details across any one 

 section are not persistent enough to determine with precision the horizons 

 over any extended area. Modoc Peak, Combs Mountain, Atrypa Peak, 

 Woodpeckers Peak, and Newark Mountain afford typical sections. 



In general the lower limestones are indistinctly bedded, light gray in 

 color, and highly crystalline, passing up into brown, reddish l)ro\vn, and 

 gray beds, which are distinctly stratified and finely banded and striped, 

 presenting a somewhat variegated appearance on the weathered sui-faces. 

 This latter feature is very persistent throughout the middle ])ortion of 

 the limestone. In the upper members the limestones are more massive, 

 usually well bedded, and possess a normal bluish black and bluish 

 gray color. In broad masses it is difficult to distinguish the upper 

 members of the Nevada limestone from the Carbc^niferous limestone, 

 and they closely resemble the gi'eat bodies of the Wasatch limestone 

 of Utah. The intercalated bands of argillaceous shale and quartzite 

 vary greatly in width, but do not especially mark any part of the 

 limestone, except that they occur more frequently in the middle portion 

 than elsewhere. Calcareous shales are found tliroughout the epoch. The 

 limestones are everywhere more or less magnesian, nearly pure dolomites 

 frequently occurring in narrow layers. At the base of the section north of 

 Modoc Peak (Fig. 3) the rock cannes 40'62 per cent of magnesium 

 carbonate, with 01 per cent of insoluble residue. In band 15, of the same 

 section, the dark colored limestone carries l"2f) per cent of carbonate of 

 magnesia, while the light coloi-ed rock holds 2G 7S per cent. 



The Modoc Section.— A sectiou in detail across the strata, extending from 

 the summit of the Nevada limestones nearly to the base, was made 

 b}' Mr. J. P. Iddings. It was constructed across the high ridge lying 

 between Signal and Modoc peaks, beginning with the lowest rocks exposed 

 at a point northwest of the latter peak just east of the ]\Iodoc fault, and 

 terminating at the eastern base of the hills where the uppermost beds pass 

 beneath the valley accumulations (atlas sheet vii). The section measures 

 5,400feet. The beds trend obliquely across the ridge, striking N. 50°-55 ° W. 



MON XX 5 



