276 IF. Lindgren — Auriferous Conglomerate of 



at this point proves that already during the Cretaceous the 

 river courses were outlined which, during the Tertiary, were 

 further eroded and then rilled with gravel. From the northern 

 part of the State Mr. R. L. Dunn has recently, in a paper 

 written for the State Mineralogical Bureau but not yet pub- 

 lished, described an interesting occurrence of auriferous river 

 gravel from underneath the Chico sandstones. 



A most marked unconformity exists at Folsom between the 

 horizontal Chico sandstones and the vertical and highly com- 

 pressed Mariposa beds ; the latter are besides cut off and meta- 

 morphosed by the adjoining intrusive mass of granodiorite. 



In view of the facts presented above it is of some interest 

 to find an auriferous conglomerate in the Mariposa beds indi- 

 cating that at least some of the quartz veins antedate the post- 

 Mariposa upheaval. 



Before describing the occurrence it may not be amiss to 

 devote a few lines to the sedimentary formations of the vicin- 

 ity. The larger part of the sedimentary area of the gold belt 

 is, as shown by the work of the U. S. Geological Survey, of 

 pre-Jurassic age. Most prominent are the Paleozoic, probably 

 mostly Carboniferous rocks and smaller belt only belong to 

 the Trias, the Jura-Trias and to the Jura. Some of the results 

 attained in the gold belt south of the fortieth parallel have 

 been set forth by Mr. H. W. Turner in a paper published 

 in the American Geologist, April, 1894. 



The locality to be described here is found in Placer County, 

 on the ridge between the North and Middle Forks of the 

 American river at an elevation of about 2100 above the sea 

 and is mapped on the Placerville atlas sheet, N.W. corner. 



Beginning from the edge of the Great Yalley there is first 

 a broad area of granodiorite, followed by a belt of diabase 

 and greenstone schists, which again are adjoined on the east 

 by a series of Paleozoic sedimentary rocks, probably all Car- 

 boniferous (Calaveras formation). They consist of black, not 

 very fissile clayslates and fine, dark colored sandstones, but 

 besides these and characteristic of the formation in this vicinity, 

 there are heavy beds and lenticular masses of crystalline lime- 

 stone very frequently changed to a black or gray chert or 

 phthanite.' The strike is nearly K-S., the dip T0°-90° E. 



Adjoining this series on the east are the Mariposa beds. 

 Through the southern part of Eldorado Co. they can be traced 

 without difficulty as a belt from ^ to 2 miles wide. At Placer- 

 ville they begin to widen and intrusive bodies of diabase and 

 porphyritic in part made schistose by dynamo-metamorphic 

 action appear in them, splitting the series into two and three 

 belts. The eastern line of demarcation towards the Paleozoic 

 beds adjoining again on the east, as laid down on Placerville 



