1918J 



Davis: The Franciscan Sandstone 



39 



rare or absent in deposits from sheet floods, and false bedding may 

 not be apparent. 



The absence of ripple marks, mud cracks, and similar features in 

 the black shales is peculiar. So far the writer has never found defi- 

 nite ripple marks on any of the black shales, nor have mud cracks or 

 rill marks been discovered. It might be possible that the shale is not 

 of the sort to preserve such surface features. Barrell has shown that 

 certain shales swell when wet by the rain and are therefore not 

 adapted for preservation of surface markings. However, the presence 

 of many angular blocks and slabs of shale in the sandstone indicates 

 that the Franciscan shale did not undergo this sort of swelling on 

 wetting after dessication. 



In the foregoing pages the conclusion is reached that the climate 

 in Franciscan time was arid. Another possibility is suggested by the 

 reference of the Franciscan group to the Jurassic period. In the 

 Mariposa slates, near Colfax, certain remarkable breccias occur. 63 

 These show numerous angular blocks embedded in a fine argillaceous 

 matrix and by analogy with other occurrences elsewhere one might 

 regard these as glacial deposits of upper Jurassic age. While no evi- 

 dence of ice action is found in the Franciscan itself, this occurrence 

 at Colfax, in connection with a possible Jurassic age for the Fran- 

 ciscan group, suggests an interesting possibility. The Franciscan 

 sandstone may be of glacial origin, representing outwash deposits of 

 sand from a glaciated region. 



While no evidence is known to warrant the conclusion that granu- 

 lar disintegration of rocks occurs in glaciated regions, it is known 

 that grains of fresh feldspar and ferromagnesian minerals occur in 

 glacial deposits. Scherzer 04 states that the grains of glacial sands 

 show little evidence of weathering or wear, being fresh, bright, and 

 sharply angular. The sands are also very poorly sorted. 



Boswell 65 has shown that the drift in northern Europe contains 

 remarkably fresh grains of many minerals, including such easily de- 

 composed minerals as amphiboles, pyroxenes, biotite, epidote, and 

 feldspars. 



Coleman 06 describes the matrix of the Keewatin tillites as an arkose 



83 C. L. Moody, Univ. of Calif. Pub)., Bull. Dept. Geol., vol. 10, pp. 383-420, 

 1917. 



G 4 Scherzer, W. II., Criteria for the Recognition of the Various Types of 

 Sand Grains, Bull. Geol. Soc. Am., vol. 21, p. 628, 1910. 



86 Boswell, P. G. H., The Petrology of the North Sea Drift and Upper 

 Glacial Brick Earths in East Anglia, Proc. Geol. Assoc., vol. 27, p. 79, 1916. 



66 Coleman, A. P., The Lower Huronian Ice Age, Jour. Geol., vol. 16, p. 152, 

 1908. 



