1918 ] Davis: The Radiolarian Cherts of the Franciscan Group 291 



OTHEE FOEMATIONS OF THE MONTEEEY GEOUP 

 Besides the terrigenous shales, bituminous shales and diatomaeeous 

 earths, other rocks are found in the Monterey group in close associa- 

 tion with the cherts. 



Limestones. — Some dense gray limestones occur. On weathered 

 surfaces this limestone may be yellowish from the presence of a coat- 

 ing of soft ocherous limonite. The beds are often lenticular and may 

 contain the bones of marine animals. They contain in addition to 

 the lime carbonate, carbonates of iron and magnesia, and phosphate 

 of lime. 



Sandstones. — There are two distinct types of sandstone associated 

 with the Monterey cherts. One of these is of the ordinary type and 

 requires no special description. The other type is rather unusual in 

 its nature and often occurs interbedded with the cherts in beds a few 

 feet thick. 



Sandstones of this type are fine grained and are generally pale 

 gray in color. The light coloration is due to the presence of an earthy, 

 white substance which occupies the spaces between the grains. An 

 occasional dark grain is seen. Often the sandstone is firm in texture, 

 but in many places it is very porous and may be broken in the fingers. 



This sandstone has been described by Professor Lawson, 24 as 

 follows : 



The microscope shows that the body of the rock is composed of fragments of 

 quartz, orthoclase and acidic plagioelase and siliceous rocks, the quartz greatly 

 preponderating. All these are remarkably angular and are embedded in an amor- 

 phous cloudy matrix which resembles the insoluble residue of the chalky shales. 

 The sharply angular fragments of quartz and feldspar are very uniform in size, 

 ranging in diameter from about 0.12 millimeters to .5 millimeters. The dark par- 

 ticles are not uniform in composition, but many of them are volcanic glass. This 

 remarkable rock, though here called a sandstone, has rather the character of a 

 quartzose tuff than of a detrital rock, and its intercalation in the cherts and 

 shales, which generally include little detrital material, supports this suggestion. 

 The question of its origin is, however, left open for the present. No fossils have 

 been found in these rocks. 



In some respects this sandstone resembles that found as a parting 

 between chert layers, described above. However, there are important 

 differences and the sandstone parting may be a distinct type. The 

 sandstone parting is brown in color and contains no volcanic glass. 



Tuffs. — There are numerous intercalations of tuff in the Monterey 

 group. One of these, found near San Luis Obispo, is 800 feet thick. 



2 * San Francisco Folio, op. ext., p. 10. 



