Jurassic Age from the Sierra Nevada. 277 



sheet, is somewhat doubtful between Placerville and the 

 northern edge of the sheet ; both at Spanish Flat and at 

 Georgetown, Mesozoic fossils have been found and the line has 

 therefore been drawn so as to include both these localities. 

 Up to the Middle Fork of the American river the prevailing 

 rock of the Mariposa beds is a black fissile slate sometimes, as 

 at Greenwood, alternating with thin beds of dark tufaceous 

 sandstone. At the latter place the contact with the underly- 

 ing Calaveras formation is well exposed with apparent con- 

 formity both dipping 70°-80° east. From here up to Colfax, 

 a distance of about 12 miles the character of the Mariposa 

 beds change. Conglomerates appear in places near the western 

 contact, the black slates become less fissile and more jointed 

 and contain abundant beds of dark tufaceous sandstones and 

 breccias ; the latter are composed of slate, phthanite and 

 limestone from the Calaveras formation together with frag- 

 ments of basic igneous rocks. Besides dikes and masses of 

 the latter cut across the strata. The -breccias appear mostly 

 conformably in the series and should probably be explained as 

 intercalated mud flows. Short distance north of Colfax a 

 massive of diabase, gabbro and diorite cuts off the Mariposa 

 beds, which here predominatingly consist of pyroclastic rocks. 

 A breccia of diabase mixed with fragments of limestone, etc., 

 extends from the massive and cuts across the beds at Colfax. 

 It is most probable that the basic intrusive area to the north of 

 Colfax represents the core of a volcano the eruptions of which 

 were contemporaneous with the deposition of the Mariposa 

 beds and the ejecta of which intermingled with the sediments. 

 The older rocks adjoining the Mariposa beds on the east 

 extend northward but to the west of these where the continua- 

 tion of the Mariposa beds should be sought no certain trace of 

 them can be found. Diorites, diabases and other principally 

 basic rocks occupy, on Smartsville sheet, nearly the whole area 

 to the west down to the valley. Both the Mariposa and most 

 of the western area of the Calaveras formation appear to be 

 engulfed in the igneous rocks if indeed the former ever con- 

 tinued to the north of Colfax. The evidence appears to favor 

 the theory that the Mariposa beds were deposited in a gulf or 

 bay, enclosed on both sides by older rocks and ending in the 

 vicinity of Colfax. In the igneous areas to the 1ST. 1ST. W. of 

 Colfax and extending up towards Grass Valley and Nevada 

 City some narrow bands and smaller areas of sedimentary rocks 

 occur. The limestone and phthanite in most of them indicate 

 that they belong to the Calaveras formation ; there is however 

 one narrow band of black tufaceous slates and sandstones, 

 between Grass Yalley and Nevada City which may possibly 

 belong to the Mariposa beds. 



