208 University of California Publications in Geology [Vol. 10 



beds of lower Etehegoin on Frame's ranch, in the upper region of 

 Jacalitos Creek. On one of the branches at the head of Waltham 

 Creek thin beds of carbonaceous material were found near the middle 

 of the formation. The coal deposits in the Etehegoin of Priest Valley, 

 however, are by far the most important both in thickness and areal 

 extent. This area has already been mapped and lithological sections 

 given by Robert W. Pack and Walter A. English. 28 



As indicated by these deposits, it seems possible that the Etehegoin 

 in Priest Valley and adjacent areas may have been laid down in a 

 basin connecting the sea in the Great Valley with that west of the 

 Diablo Range. That such connections probably existed has already 

 been pointed out by F. L. Ransome, 29 who states: "The Great Valley 

 was probably occupied during the whole of the Neocene by a gulf, 

 connecting with the ocean by one or more sounds across the Coast 

 Ranges. 



KHYOLITIC TUFFS 

 Effusive rocks have been found at three horizons in the Etehegoin. 

 These consist of rather persistent tuffaceous bands interstratified with 

 other rocks of clastic origin. From the uniform thickness of each bed, 

 the stratification planes often being distinctly visible, and from the 

 intimate relation to strata containing a marine fauna, it is evident 

 that these rocks were deposited in the waters of a shallow sea. The 

 fine, well-sorted character indicates that the volcanic action must have 

 taken place at a considerable distance from this area, although prob- 

 ably in the Diablo Range, along the foothills of which the Etehegoin 

 outcrops. 



On approaching Jacalitos Creek from the Stone Canon coal mine 

 to the west a prominent white bed or band can be seen on the eastern 

 bank several miles before reaching the creek. This is the lowest of 

 the three horizons indicative of volcanic action in the Pliocene of this 

 region. The bed is about twenty feet thick and is exposed for nearly 

 two miles along the strike until cut off by faulting. Petrographic 

 examination shows that quartz and orthoclase feldspar are the pre- 

 dominant minerals in this rock. Plagioclase feldspar is also present, 

 but in minor proportion. The composition therefore corresponds to 

 that of a rock usually grouped under the term rhyolitic tuff. 



28 Pack, Bobert W., and English, Walter A., Geology and oil prospects in 

 Waltham, Priest, Bitterwater, and Peachtree valleys, California, U. S. Geol. 

 Surv. Bull. 581D, 1914. 



2» Eansome, F. L., The Great Valley of California, a criticism of the theory 

 of isostasy, Univ. Calif. Publ. Bull. Dept. Geol., vol. 1, p. 386, 1896. 



