FEATURES OE THE QUARTZITES. 43.'] 



ing quartzites are of two kinds; in one the .siliceous rock retains the 

 slaty felting and in part the slaty lamination, and this quartzite may be 

 described as silicated slate ; in the other kind the felting and lamination 

 have disappeared and the siliceous mass is often partially or completely 

 oolitic. The one kind passes into the other by gradation, sometimes 

 within a few feet. There are no sandstones among the slates in this 

 district, and I conclude that the difference is due to different kinds or 

 degrees of alteration, and not to original differences in the sediment of 

 which the rock was composed. The quartzites frequently pass by farther 

 alteration into clear, white massive quartz. The quartzite is commonly 

 dark gray when freshly exposed, but weathers to some shade of yellow 

 or red from oxides of iron, and then to gray. 



Limestones. — There are limestones in these slates, as shown on the 

 sketch map (plate 13). They replace quartzites in the line of strike and 

 are otherwise so associated with quartzites as to indicate that the latter 

 have replaced the limestones, but lithological examination is necessary 

 to determine definitely whether this is so. 



MESOZOIC ROCKS OUTSIDE OF UPPER FEATHER RIVER DISTRICT. 



Distribution of the Rocks. — The greater part of the Mesozoic exposures of 

 the range are included within two principal areas, an eastern and a 

 western one. The eastern and larger one begins at the northern end of 

 the range and there includes its eastern crest, and extends in width west- 

 ward to the western pre-Mesozoic area, as shown on the sketch map 

 (plate 13). Farther southward it has the eastern pre-Mesozoic area on 

 the east, and lies between it and the western pre-Mesozoic area, and con- 

 tinues so to the southern end of the latter. There it lies between the 

 eastern pre-Mesozoic exposures and the unaltered Tertiary deposits of 

 the valley for the greater part of the distance to its southern end. which 

 is about I5miles southeast of the Merced, where the pre-Mesozoic rocks 

 come forward to the Great valley. Three minor arm- of pre-Mesozoic 

 exposures already mentioned lie between it and the Tertiary of the 

 valley, one 1 iet ween the ( a 1 a vera s and Stanislaus, one on the Stanislaus, 

 ami one south of the Merced aboul Hornitas. 



The western principal area of Mesozoic exposures lies along the western 

 font of the range, between the ^ estern area of the pre-Mesozoic exposures 

 and the unaltered upper Cretaceous and Tertiary rocks of the Greal 

 valley, and extends southward from the northern end of the range to 

 where the granite of the western granitic area comes forward to the valley 

 between the Yuba ami American rivers. 



Fossiliferous lower Mesozoic Limestones. — I have not seen the laminated 

 slates of the head of the series in the we- tern area, though there may he 



