Notes on the chemical geology of the gold-fields of California / by J. Arthur Phillips.

  • Phillips, J. Arthur (John Arthur), 1822-1887.
Date:
[1868]
Catalogue details

Licence: Public Domain Mark

Credit: Notes on the chemical geology of the gold-fields of California / by J. Arthur Phillips. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    spar, and mica, the latter of which is well crystallized and often of a brilliant black colour, this rock incloses titaniferous iron, and occasionally crystals of sphene of a light yellow colour. Magnetic oxide of iron is also present in notable quantities, and consequently imparts this property to the enclosing granite. The granite of the Sierra appears to retain its peculiar character- istics throughout a great portion of its extent; but towards the southern extremity of the chain hornblende begins to make its appearance as one of the constituents of the rock; the mica at the same time becoming less plentiful, and losing its brilliancy and dark colour. Lying between the band of metamorphic slates and the great central granitic mass forming the more elevated portions of the chain, are found various eruptive rocks, such as syenites, diorites, and porphyries. The first of these pass into granites by almost imperceptible gradations, and appear to belong to the same geological period. The diorites, on the contrary, are of more recent origin, and contain neither sphene nor titaniferous iron, although they enclose a considerable amount of magnetic oxide of iron. In many places diorite is observed to traverse the granite in the form of distinct dykes. The porphyries are com- paratively of small extent, and contain crystals showing the usual strise, indicating the presence of plagioclastic felspars. Quartz Veins. All the crystalline rocks in the vicinity of the sedimentary deposits contain numerous veins of quartz, which traverse the granite and diorites near their point of junction with the slates, and, in addition to gold, enclose crystals of iron pyrites and other metallic sulphides. These quartz veins of the crystalline rocks are comprised within a narrow zone running from south to north, along the western flank of the mountains above the great band of meta- morphic slates forming the most productive portion of the gold- region, and extend, in the vicinity of the line of junction, nearly throughout its whole extent. The quartz veins of the band of metamorphosed slates occu- pying the western slope of the Sierra, and lower on its flank than the crystalline rocks above described, are numerous and important. They are not, however, by any means equally dis- tributed throughout the region of slates, but are chiefly con- centrated in a belt having a width from east to west of some twelve or fifteen miles, and extending from south to north throughout the whole length of the formation. These veins, for the most part, follow the general direction of the strata in which they are enclosed. This parallelism, however, is not A 2