64 



University of California Publications. 



[Geology 



SONOMA TUFF. 



Following the andesite came enormous quantities of volcanic 

 ashes. These ashes covered a large area, part of them falling 

 upon dry land, part upon water, and part being washed down by 

 the streams to form tuffs in the seas and lakes. 



Character of the Tuff. — This tuff is a fragmental rock made 

 up wholly of the volcanic material, and characterized by con- 

 taining numerous fragments of pumice, in size from very small 

 grains up to an inch or more in length. Two silica determina- 

 tions made on the pumice, from two localities in Santa Rosa Val- 

 ley, gave respectively 61% and 63% SiCv Hence it is ande- 

 sitic in character. 



The rock is usually very light in color and in weight, and, 

 where well exposed, forms a conspicuous feature of the land- 

 scape. Certain fine grained varieties of it are easily worked into 

 blocks, which make very good building stone where great strength 

 is not required. 



Areal Distribution and Thickness. — It is widely distributed 

 in the Coast Ranges north of Mt. Diablo for at least 100 miles, 

 and, as it is easily recognized, it was the writer's idea at one 

 time to use it as a datum plane in an attempt to trace out the 

 late Tertiary basin of the region. It was soon found, however, 

 that this would take more time than was available, and the orig- 

 inal plan of a continuous cross-section through a typical locality 

 was resumed. 



Good exposures of this tuff may be seen from Freestone south- 

 east to Bloomfield, near Trenton and on the Russian River near 

 the mouth of the Mark West Creek, in the lower foothills north- 

 east of Windsor and near Mark West Springs as shown in Sec- 

 tion AB. From here east to Calistoga, and beyond to the high 

 ridge of Twin Peaks, there are large areas and immense thick- 

 nesses of it. As shown by Section CD, it is much thinner to the 

 south. In fact, it was necessary at many places to exaggerate its 

 thickness to show it at all on the section, since toward the south- 

 west part of the area it is sometimes not more than ten or fifteen 

 feet thick. From Mark West Creek to Franz Valley and at Twin 

 Peaks southwest to Oat Hill Quicksilver Mine it has its greatest 

 thickness, about 1,700 feet. Toward the west side of Santa Rosa 



