1918] 



Davis: The Franciscan Sandstone 



21 



lization. Occasionally a few flakes of sericite appear within a feld- 

 spar grain, or a veinlet of sericite cuts into it. Sometimes the whole 

 periphery of a feldspar grain will be rimmed with sericite. For the 

 most part, however, these effects are unimportant. No evidence of the 

 shearing of the clastic grains is visible. 



In attempting to distinguish the nature of the cement of the sand- 

 stone, one is handicapped by the extreme fineness of the grains which 

 compose it. In a few instances the cement appears, as noted above, to 

 be composed in large part of finely broken mineral fragments which 

 have been little affected by recrystallization. 



In an occasional specimen, calcite is an important constituent of 

 the cement, but for the most part it is present in only a few scattered 

 grains. These are visible only under the microscope, but their pres- 

 ence is shown by a slight effervescence when the sandstone is treated 

 with acid. 



Minute shreds and flakes of a clear, colorless mineral are also seen. 

 This shows a moderate relief, moderately high birefringence, parallel 

 extinction and positive elongation. It is determined as sericite. In 

 places it is quite abundant in. the cement, occurring as aggregates of 

 scales and flakes. At other places only a few shreds appear, separated 

 from one another by the minerals of the cement. 



A few small flakes of a green mineral also occur in the cement. 

 Prom their peculiar interference colors, pleochroism, color and form, 

 they are determined as chlorite. The pale greenish color of the cement 

 is probably due to very finely divided chloritic material. On the alter- 

 ation of the rock, the first change which occurs is a slight discolora- 

 tion of the cement from pale green to yellowish, suggesting the oxida- 

 tion of some mineral containing ferrous iron. In this stage the cement 

 is seen, in thin section, to be colored yellowish brown by some very 

 finely disseminated substance not clearly visible under the microscope. 



In certain specimens, especially those in which only a little cement 

 is present, and the grain of the cement is very fine, the latter appears 

 to consist wholly of sericite with a little chlorite. However, the great 

 strength of the cement indicates the presence of something else. It is 

 probable that the polarization colors of the sericite mask the presence 

 of silica having a low birefringence. This appears probable from the 

 fact that where the cement is more coarsely crystallized the sericite 

 crystals are intermixed with secondary quartz and with a mineral of 

 low relief, low birefringence and fibrous extinction that is regarded 

 as chalcedony. 



