18 University of California Publications in Geology [ VoL 11 



Mount Diablo. Fairbanks 40 reports peculiar pecten-like forms from 

 one locality in the Franciscan sandstone of the San Luis Quadrangle. 



This lack of fossils is also a characteristic feature of the black 

 shales of the Franciscan. With the exception of some peculiar mark- 

 ings that resemble Avorm tracks, no fossils have been found in this 

 shale except those discovered in the slates at Slate's Springs. 



Carbon is fairly common in the sandstone. It occurs in the form 

 of pieces of carbonized wood that often show the woody structures, 

 also as fragments of leaves. The sandstone also contains a few very 

 thin lenses of coal. These have been noticed at many localities 

 throughout the Franciscan terrane. So far, no one has discovered 

 identifiable plant remains save at Slate's Springs. In the neighbor- 

 hood of the seams of coal the sandstones lose their usual hard, well 

 cemented character and are little indurated. 



Lithologic Character 



When fresh, the typical Franciscan sandstone is dark greenish 

 gray to bluish gray in color. It is frequently cut by small veins of 

 quartz and occasionally by veinlets of calcite. The texture is dense 

 and the grains are closely packed. The strength of the cement is 

 shown by the fact that, in breaking the rock, the fracture passes 

 through the mineral gi'ains as readily as through the cement. In this 

 respect, the sandstone is very much like a quartzite. 



In general, the sandstone is medium coarse grained. The diam- 

 eters of the grains range from a quarter to three-quarters of a milli- 

 meter. Occasionally there may be a small lens or pocket of sandstone 

 in which all the grains are coarse, ranging from a millimeter up to 

 three millimeters in diameter. 



In hand specimen, quartz, feldspar, muscovite, and biotite are 

 easily recognized. With a hand lens, the striations on the grains of 

 plagioclase are often clearly visible. Numerous small angular blocks 

 and flakes of black shale are present in the sandstone, and frequently 

 fragments of serpentine are found. 



The sandstone readily decomposes under the action of the weather 

 and it is usually covered with a deep soil. The process of alteration 

 begins with a discoloration of the rock; it rapidly loses its bluish or 

 greenish color and becomes first yellowish and then brown. At this 



40 Fairbanks, H. W., San Luis Folio, U. S. Geol. Surv. Folio no. 101, 1904. 



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