442 J. E. MILLS — ROCKS OB' THE SIERRA NEVADA OF CALIFORNIA. 



south of Merced river, about Hornitas, have also yielded considerable 

 quantities of gold. 



Gold lias been found in all the members of the Mesozoic series except- 

 ing the serpentine; but much the most productive part of the series and 

 of all the rocks in the Sierra is what I have called the lower Mesozoic 

 subgroup, which includes the slates and greenstone- at the bottom of the 

 Mesozoic series and the slates and limestones and greenstones next above. 

 Nearly all of the deposits now most largely productive are in this part 

 of the Mesozoic series, excepting some in fissures and dikes, which, 

 though traversing older rocks, probably tor reasons already given belong 

 to the same Mesozoic age. As the lodes are generally if not always at 

 fault planes, they are < >ffcen at or near contact of this sul (group of Mesozoic 

 rocks with others of a widely different horizon, as at Nevada city, where 

 the contact is with granite. 



Professor Whitney describes the before-mentioned great quartz vein, 

 called the "Mother lode,'' extending (not continuously) from the Mari- 

 posa estate to Amador county, as " Made up of irregularly parallel plates 

 of white compact quartz and crystalline dolomite or magnesite." * There 

 is a large vein in the greenstone-bearing group on the southeastern Hank 

 of Spanish peak mountain, which also consists largely of magnesian lime- 

 stone. The " Mother lode '" between the Calaveras and Tuolumne rivers 

 and also in Mariposa county south of the Merced, if not for its whole 

 length, is at the head of the lower Mesozoic subgroup. This is the hori- 

 zon of the fossiliferous limestones, and it is possible that the limestone of 

 the lode where it occurs belongs to this group of sedimentary deposits, 

 but it is also possible that it is a chemical deposit like the quartz. 



The fact that the most profitably worked quartz deposits are in the 

 lower Mesozoic subgroup does not prove that the rocks of that subgroup 

 contain the most gold, but that they contain it in the form most avail- 

 able. In the other Mesozoic rocks (excepting the serpentine) and in the 

 pfe-Mesozoic sedimentary rocks there must be much gold in a more dif- 

 fused condition, for the gravels which are debris of these rocks are often 

 very rich. But I cannot here treat of the occurrence of gold further 

 than as it is characteristic of the geology of the series and its several 

 members. 



Fissures containing Chalcopyrite. — In a long line of fissures near the 

 western foot of the range, chalcopyrite occurs with the quartz and pyrite 

 as an important constituent of the vein matter. The fissures are among 

 or near the greenstones of the lower part of the Mesozoic series. Such 

 deposits occur on the southern side of the Yuba, near Spencerville ; south 

 of Sutter creek, about 21 miles south of east of lone ; on the Mokelumne, 



* Auriferous Gravels of the Sierra Nevada. 1879, ]>. 46. 



