1918] Davis: The Franciscan Sandstone 37 



SHALE BLOCKS AND SLABS 



The presence of numerous shale blocks and slabs in the conglomer- 

 ates is opposed to the idea of their marine origin. Such angular frag- 

 ments of shale could not be carried very far without breaking up, 

 nor could they withstand the action of waves on a beach for any 

 length of time (plate 2a). 



Masses of tough clay are not uncommon on the sand beaches south 

 of San Francisco, where they appear to have been torn off clay banks 

 beneath the water since they show the perforations of boring mollusks. 

 Such masses, if embedded in the shore deposits, would give rise to 

 shale boulders upon consolidation. Grabau r ' 8 describes similar exam- 

 ples. However, these masses of clay are not like the shale boulders 

 which occur in the Franciscan sandstone. The clay masses on the 

 beaches are invariably rounded by beating against the shore. The 

 shale blocks of the Franciscan are angular, showing sharp corners 

 which indicate brittleness at the time of their formation. They show 

 bedding planes marked out by sand layers. These are truncated by 

 the sides of the block in such a way as to indicate that the mass was 

 consolidated into shale before breaking. The clay masses on the 

 present beaches hold together fairly well under the action of the 

 waves because of the toughness of the plastic clay. Hardened shale, 

 however, is brittle and would be easily broken down in a beach zone 

 by the action of the waves, and the alternate drying and wetting. 



The presence of isolated blocks of shale with angular boundaries, 

 embedded in the midst of massive sandstone, is not explainable on the 

 hypothesis of the marine origin of the sandstone. If, however, the 

 sandstones were deposited by the action of streams, in the manner 

 above suggested, then the blocks may be due to a process observed by 

 Blanford and Theobald 59 in certain rivers of India (the Brahmini and 

 Mahanuddi). During the rains much sand is brought down by these 

 rivers and deposited on their flood plains. In the dry season the rivers 

 retire into small channels and leave large bodies of water, charged 

 with fine mud, in the river bottoms. As the water evaporates, the 

 mud becomes hardened, cracks, and breaks into blocks. A sudden 

 influx of water in a freshet may cause the shale beds thus formed to 

 be broken up and their fragments embedded in the newly deposited 

 sand. Such behavior explains the breaking up of shale lenses with- 

 out perceptible planes of faulting or shearing in the sandstone 

 (%• 5). 



58 Ibid., p. 711. 



■■■■> Mem. Geol. Surv. India, vol. 1, pp. 52-53, 185!). 



