University of California Publications. Bulletin of the Department of Geological Sciences Volume 11 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1918 Excerpt: ...of foraminifera, showing in many cases the delicate details of the external form of the shells. In some instances the lime carbonate of the shells is still present, not having been leached out. Under the microscope these white shales are found to consist largely of opaline silica in which there are very small amounts of argillaceous matter together with a few minute, angular grains of quartz, and an occasional grain of feldspar. These mineral grains may make up as much as five per cent of the volume of the rock. "When the lime carbonate of the foraminiferal shells is still present it is seen to be in the form of fibrous aragonite. The minute perforations of the shell appear clearly in such instances, and the chambers are usually filled with crystalline calcite. In most of these white opaline shales there are no other organic remains. By treating the diatomaceous shales and the white opaline shales with caustic potash, amorphous silica gocs into solution to an amount from fifty to seventy per cent. After dissolving the opaline or amorphous silica the residue consists of minute grains of quartz, feldspar and various other minerals, together with some argillaceous material. In addition to these substances there are often numerous fragments of volcanic glass. In some cases a large proportion of the rocks consist of this material so that gradations exist between the typical shales and acid tuffs. MONTEREY CHERTS In addition to the shales, siliceous rocks of a cherty nature are found in the Monterey generally at definite horizons. The cherts of the Monterey group may be divided into two types: the bedded cherts and the flinty cherts. The bedded cherts are like the cherts of the Franciscan in that they are rhythmically interbedded with shales. The flinty cher...