Louderback, G. D. "Benitoite, Its Paragenesis and Mode of Occurrence." University of California Publications.
Page 366
The writer inclines to the belief that this rock was originally a facies of the Franciscan radiolarian chert. He has seen dis- tinctly altered cherts that have a somewhat similar texture and mineral appearance under the microscope. The Franciscan cherts grade over insensibly into siliceous iron ores and in a number of localities have associated with them deposits of man- ganese dioxide. This would explain the high iron and low alumina content and the association with manganese stringers. Much or all of the soda and other oxides in part may have been introduced during the metamorphism, as in the case of certain crocidolite schists of the Coast Ranges which the writer has found to have been derived from ferruginous cherts by a similar process. [18]

Of the rocks described as associated with the veins the green- stone (altered diabase) is the most abundant and the one most commonly in contact with the veins in moderately altered con- ditionÑespecially towards the east end. On approaching the central part of the zone of veination, however, the alteration in- creases very greatly, the original pyroxenic constituents disappear and the chief constituents are the new-formed amphiboles. The old structures are entirely lost. In part we may refer to the material as soda-amphibole schist.

A still further alteration is caused by the leaching out of the felclspathic constituents, leaving the rock in a more or less porous condition, as occurs on the left side of the cut shown in plate 32.

This rather porous rock near the veins may be thoroughly impregnated with natrolite for a fraction of an inch or several inches from the vein; also it is in this rock that the spaces occur covered with free-growing amphibole needles on which the natrolite groups are perched as already described.

SEQUENCE OF EVENTS.

The field relations and lithologic characteristics indicate that the rocks in which the benitoite-bearing veins occur are a detached mass of the Franciscan series, showing both igneous and sedimentary facies, that was included in the serpentine at the time of its intrusion.

NOTES:
18 Louderback and Sharwood: Bull. Geol. Soc. Am., 18 (1906), abstract p. 659.
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