424 J. E. MILLS — ROCKS OF THE SIERE.A NEVADA OF CALIFORNIA. 



Mesozoic rocks continue west of the limestone on the Mokelumne about 

 eight miles. From this exposure of limestone on the Mokelumne to the 

 most southerly one described by Whitney is about 40 miles. These 

 gneisses, mica-slates and limestones underlie unconformably strata known 

 to be Mesozoic, but no fossils have been found in them and their age is 

 not definitely determined. 



About midway between the Calaveras and Stanislaus rivers in the 

 Great valley, about three miles west of its eastern edge, is a small area of 

 granite. It adjoins Mesozoic rocks on the east and passes westward 

 under Tertiary deposits. It suggests an extension of pre-Tertiary uplift- 

 ing of the western division of the range far south of the Tertiary and 

 Quaternary uplifting of that part of the range. There is an area of pre- 

 Mesozoic gneisses and other rocks between the Mesozoic outcrops and the 

 Tertiary deposits of the valley on the Stanislaus ami a much larger one 

 south of the Merced, about Hornitas. The eastern area of granite conies 

 forward to meet the Tertiary of the valley near where the San Joaquin 

 cumes ont of the mountains/'- and only isolated areas of sedimentary 

 rocks are found on the western flank of the range farther southward. 



Age of the Pre-Mesozoic Rocks. — -I have treated the pre-Mesozoic rocks 

 as of one group. It is not proven that they are all conformable or all of 

 one period. It is entirely possible that a part of them are Arehean and 

 a part Paleozoic, and that the latter part may include rocks of different 

 Paleozoic periods. Indeed, there remains a remote possibility that some 

 of them may he early Mesozoic. older than the oldest group that is proved 

 to he Mesozoic; but they are much more metamorphosed than these, are 

 unconformable with them, and after having been deposited were certainly 

 metamorphosed and uplifted, and the region had begun to subside again 

 before the lowest known Mesozoic strata were deposited. It is not there- 

 fore within reasonable probability that any of these rocks are later than 

 Paleozoic. 



Besides being altered and tilted and faulted, the sedimentary rocks of 

 this group are very widely overlain by Mesozoic rocks, and their outcrops 

 are consequently disconnected; and fully to determine their order of 

 succession will require examination and comparison of a large part of 

 the areas of their exposure in the Sierra. The Mesozoic rocks, on the 

 other hand, are not overlain except by comparatively thin Tertiary and 

 Quaternary deposits, and therefore their sequence and natural division 

 into subgroups are more readily determinable in spite of faulting, tilting, 

 overturning and metamorphism. The district represented on the accom- 

 panying sketch map (plate 13) is a typical one for these rocks. 



* According t<> ma]' by Win. P. Blake in his "Geological Reconnoissance in < lalifornia," 1853. 



