1917] 



Moody : 



Breccias of the Mariposa Formation 



409 



to have supplied the silty material that now forms the slates, and 

 much of the less triturated clastic that is comprised in the sandstones. 

 The lateral boundaries of the basins, presenting such bold relief, would 

 be vigorously attacked by the forces of weathering and their shoreward 

 slopes would furnish abundant rock-waste which, accumulating at 

 their base, would spread out as alluvial fans, the outer margins passing 

 well under the shallow waters of the adjoining arm of the sea. 

 Weathering in a humid climate produces fine-grained material nor- 

 mally which approaches the texture of clay ; if feldspathic rocks are 

 being reduced, much kaolin may indeed be mixed with the sandy mass. 

 Siliceous rocks however are apt to fracture into angular pieces, which 

 in time of maximum precipitation, travel down the slope and become 

 intermingled with the finer material produced more slowly ; igneous 

 rocks likewise tend to fracture and split into angular fragments, and 

 they too become part of the growing fan. Rapidly growing high-grade 

 consequent streams developed in the degradation of a steep front, soon 

 cut back by headward erosion far enough so that material transported 

 from the headwaters, by the time it has reached the lower courses, 

 shows considerable rounding, while in times of freshet, subangular 

 fragments are supplied in abundance. Here, then, is a mechanism 

 which furnishes a heterogeneous mixture of coarse material grading 

 from wholly angular to well rounded, with a silty to sandy matrix. 

 Progressive subsidence of the floor of the basin of deposition must 

 have occurred to accommodate the very considerable thickness of 

 sediments. If this progressive lowering of the basin took place in 

 stages, as, for example, by movements along a zone of faulting, or by 

 intermittent down-folding on a monocline, the frequent alternations 

 from coarse to fine sediments observed within the series are readily 

 accounted for. 



The conditions of deposition along the eastern margin of San 

 Francisco Bay present helpful analogies with the conditions which 

 may have existed during the filling up of the Mariposa troughs. A 

 fault-zone, known as the Haywards Rift, separates a region of con- 

 siderable relief from the comparatively low-lying plane upon which 

 Berkeley is situated. Angular material is furnished by the rapid 

 degradation of the Berkeley Hills, while the Sacramento-San Joaquin 

 river systems supply the bay with the silty material which may in 

 time become consolidated into shale or even slate. If the present land 

 surface west of the fault-zone were lowered slightly below the bay- 

 level, conditions would be realized in which fine material could mingle 



