1918] 



Davis: The Franciscan Sandstone 



V.) 



stage the cement is considerably weakened. On breaking such a dis- 

 colored sandstone, the grains pull out of the cement instead of frac- 

 turing as they do in the fresh rock. As the change proceeds, the rock 

 becomes softer and is cut by large numbers of fracture planes, closely 

 spaced and rather irregular in their distribution. These are probably 

 due to volume changes involved in the oxidation and alteration of the 

 rock minerals. The fresh rock shows none of this sort of fracturing. 

 In the late stages of the alteration, the sandstone becomes so soft that 

 it can be easily crushed in the fingers. In the final stage a sandy 

 soil results. 



When the rock begins to alter so that the cement is weakened it is 

 easily recognized as a sandstone ; but in the fresh state, the denseness 

 of texture, color, mode of fracture, and the presence of fresh feldspar 

 cause it to resemble an igneous rock. Sometimes, at first glance in 

 the field, one will mistake the sandstone for some of the fine grained 

 gabbroid rocks of the Franciscan group. The presence of quartz 

 grains in the sandstone serves, of course, to distinguish it from these 

 rocks. 



In thin section, the clastic nature of the rock is at once apparent. 

 When first examining it, in thin section, one is surprised at the marked 

 angularity of the grains that compose it. Very few of the grains 

 show any evidence of rounding ; the greater proportion of them show 

 sharp angular boundaries. Many of the grains have delicate points 

 on their corners. Wedge-shaped pieces are common and occasionally 

 there are thin splinters with fine points. It is evident that there was 

 no great amount of abrasive action in the transportation of the grains 

 of this rock to their place of deposition. 



In some instances the closeness of the spacing of the grains is 

 notable. They fit very closely together with only a thin film of cement 

 between them. In other instances the grains are more widely sep- 

 arated and there is considerable interstitial material. 



The mineral composition of the clastic grains is also rather unusual. 

 Quartz is the most important constituent of the rock. Next in order 

 are grains of feldspar. These frequently make up a third to a half of 

 the sandstone. Grains of orthoclase and plagioclase are numerous, 

 but microline is rarely present. The plagioclase feldspars are usually 

 of the more acid varieties. As a rule the feldspars are very fresh ; in 

 unaltered specimens of the sandstone, the feldspars are as clear and 

 limpid as the quartz. In a few instances they are slightly clouded by 

 decomposition. 



