276 
GEOLOGY. 
of the principal erupted axes of the region, and is the first rock met with on leaving the plains 
of the Sacramento for the interior. 
The observations upon the outcrops of the granitic and metamorphic rocks along the line of 
the expedition, in the Valley of the San Joaquin and the Tulares, indicate that there is a suc¬ 
cession of granitic axes, trending nearly northwest and southeast, and thus making a consider¬ 
able angle with the main axis of the Sierra Nevada. These successive belts of metamorphic 
strata and erupted granites, and the like, pass below the more modern deposits of the great 
California Valley, as will he seen upon the General Map. It will probably he found that the 
metamorphic axes, or the lines of uplift and flexure, throughout the Sierra Nevada, trend 
obliquely to the general axis of the chain. This view is sustained by the topography of the 
Coast Mountains and other chains. 
The next important formation is the white, crystalline limestone, which was found developed 
to such an extent at Columbia, Sonora, Cave City, and the vicinity. It attains a great width ; 
is nearly vertical in its planes of structure, and appears to form a long belt trending north¬ 
westerly through Tuolumne and Calaveras counties. It probably extends much further, both 
north and south. It is believed to be parallel to, and conformable with, the great slate formation. 
At Abbey’s Ferry, on the Stanislaus, it was found in connexion with mica-slate and granite. 
This rock is doubtless metamorphic, but as no fossils or relics of any have been found in it, it is 
impossible to decide upon the period to which it should be referred. It is possibly carboniferous 
limestone , this formation having been recognized in the northern part of the State, while we 
are yet without evidences of the presence of Silurian formations west of the great central chain of 
mountains. 
The rock is very different from the limestone of the Tejon Pass and the Canada de las Uvas, 
being more regularly stratified, and having many blue veins and layers, all trending with the 
beds. The Tejon limestones are compact, granular marbles, perfectly white and containing 
scales of graphite ; the metamorphic action having been more intense than upon the limestone 
of the gold region. 
If a line be drawn through the several known outcrops of this limestone, it will he found to 
extend in a northwesterly direction, and if prolonged, would intersect the slate region in the 
region of Coloma, Auburn, or Yankee Jim’s. Limestone is not known to occur there, and it is 
probable that nearly opposite to Mokelumne Hill the outcrop is deflected to the east. It is at 
this point, or abreast of it, that the trend of the Sierra Nevada becomes greatly changed; its 
direction becoming nearly north and south rather than northwest and southeast. All the 
formations—slates, limestone, and the granite—appear to be shifted further east, and to pass 
behind the formations south of the American or Cosumnes Rivers. The possibility that the 
great limestone belt may pertain to the lower or western granitic axis between Sacramento and 
Auburn is worthy of consideration, but in the present state of our knowledge of the region, and 
the absence of accurate maps, it is impossible to arrive at any satisfactory conclusion respecting 
the relative positions of the formations north and south of a line drawn east and west through 
Carson’s Pass, at the point where the change in the direction of the Sierra Nevada commences. 
These formations—the erupted and metamorphic rocks—form the floor or bed-rock upon which 
a very different series of formations is deposited. These formations consist of the auriferous 
drift in its various forms, and of a more uniform and extended series of nearly horizontal strata 
of clays, sand, and gravel. These last are of marine origin, and probably Miocene or Pliocene 
Tertiary. In many parts of the region they are entirely swept away, and scarcely a vestige 
