Vol. 4] Osmont. — Geological Section of Coast Ranges. 



67 



cernable to the naked eye, some of them being 2 mm. in length. 

 They appear to be wholly feldspar. The ground mass is unre- 

 solnble to the naked eye. 



Microscopically it is strikingly similar to the rock described 

 from Mark West Springs. The two periods of consolidation are 

 even more sharply divided, and the movement of the magma be- 

 tween the two periods is still more clearly shown. The feldspars 

 gave 35° as their maximum extinction angle, indicating a labra- 

 dorite of composition near Ab 3 An,. Large lath shaped labra- 

 dorite phenocrysts were embedded in a microcrystalline ground 

 mass made up of a multitude of very minute microlites of labra- 

 dorite and grains of augite in intersertal structure. No olivine 

 was observed. A small amount of brown glass is present, which 

 seems to have come from some exterior source. This rock would 

 be called ordinary basalt. 



Age. — Near Freestone numerous casts of shells were found 

 in this tuff, but they were not determinable, though they were 

 certainly marine in character and resembled the shells of the 

 Merced above. Its conformable relation to the Wilson Ranch 

 beds suggests its being of lower Merced age. 



MARINE BEDS OF WILSON 'S RANCH. 



Lithological Character. — Near Wilson's Ranch, on the Rus- 

 sian River, some two miles above the mouth of Mark West Creek 

 and lying conformably upon about 100 feet of Sonoma Tuff 

 interbedded with coarse volcanic conglomerate and yellow sand- 

 stone, is a formation of heavy-bedded, friable yellow to buff col- 

 ored sandstone, interbedded occasionally with fine volcanic con- 

 glomerate and beds of shells. It dips gently toward the Santa 

 Rosa Valley, and is of unknown thickness, but is probably up- 

 ward of 2,000 feet. On account of their soft, friable nature these 

 beds weather in characteristic "Bad Land" forms. 



Areal Distribution. — These beds extend along the east bank 

 of the Russian River, apparently about half way to Healdsburg, 

 dipping to the east and disappearing under the valley alluvium. 

 On the eastern side of the Santa Rosa Valley, exposed along the 

 foothills north of Mark West Creek, are sandstones of very simi- 

 lar appearance and weathering, but interbedded with much 



