1918] 



Davis: The Franciscan Sandstone 



2:\ 



The low silica, indicating the presence of only moderate amounts 

 of free quartz, is the most striking feature of the analyses. The high 

 alumina and alkalies indicate the presence of much feldspar. The 

 content of soda and lime is decidedly different from that of the ordi- 

 nary sandstone. The low content of carbon dioxide shows that 

 the lime is largely contained in the plagioclase feldspars. The per- 

 centage of ferrous iron in this rock is in contrast with that in the 

 ordinary sandstone and is due to the presence of undecomposed ferro- 

 magnesian minerals. 



Reference has already been made to one of the striking features 

 of the Franciscan sandstone. This is the presence of considerable 

 numbers of small angular fragments of shale. Also in many places, 

 there are large numbers of flat flakes of shale (plate 1a). In some 

 cases these shale flakes are so numerous that when the rock is slightly 

 weathered it takes on a fissile character. They may make up as much 

 as 10 per cent by volume of the rock. 



The sandstone also contains angular blocks of shale ranging in 

 dimensions up to a couple of feet (plate 2a). These angular blocks 

 occur both in the local lenses of conglomerate and entirely alone in 

 the midst of massive sandstone. At Point Richmond, sections may be 

 seen where numerous isolated angular blocks of shale, up to a foot in 

 diameter, lie in the midst of a fresh massive sandstone of moderately 

 coarse grain. The resemblance of the sandstone to an igneous rock, 

 together with the scattered blocks of black shale, gives one the impres- 

 sion of baked shale fragments included in an intrusive rock. While 

 these shale blocks are often angular they are more commonly sub- 

 angular, the corners being worn down slightly. The material of which 

 the blocks are composed is often striped with thin layers of sand in 

 exactly the same manner as ordinary Franciscan shale. These bands 

 are truncated by the boundaries of the block in such a way as to 

 prove that the 'shale was hardened first and then later embedded in 

 the sandstone as a fragment (fig. 3). In one section, seen in the 



Shale Fragments 



Fig. 3. Block of shale in Franciscan sandstone 



