WEST COAST PULMONATA. 505 



enough for present purposes. Near the boundary of Marin 

 County some of the hills are only about three hundred feet 

 in height, allowing the sea breeze to pass inland with almost 

 as much force as at San Francisco Bay, and their desicca- 

 ting effect in summer is shown by the absence of trees over 

 most of the west slope as far north as the low depression 

 extends. But a little north of Bodega Bay the tertiary sand- 

 stones begin to extend over the metamorphic rocks farther 

 inland, rise higher, and accompanied by a dense forest of 

 redwood, etc., soon covering almost the whole surface of 

 the country near Russian River. The fossils are numerous 

 in this sandstone in many places, being as late as the plio- 

 cene epoch along Mark West Creek, twenty-five miles in- 

 land. The pulmonates of Marin County here become more 

 numerous, extending north throughout the whole width of 

 Sonoma County (excepting limited portions of metamorphic 

 rocks), with increase of numbers, size and perfection. Nos. 

 10, 27, 29, are apparently varieties produced by improved 

 conditions, especially increase of moisture, lime, shelter, 

 and vegetation suited to their natures. There is no doubt 

 tliat close search would reveal many others of the bay list 

 there, if not new forms, the smaller kinds being slowl}^ dis- 

 covered. Among them. No. 28 is likely to be found living. 

 Fort Ross, in the northwest corner of the map, is the 

 locality where Nuttall obtained some of the types, wrongly 

 credited to San Diego. 



Na]L)a County. 



The first appearance of volcanic rocks in any great amount 

 has been mentioned as occurring on the northeast slope of 

 Marin County, and they reappear in abundance on Sonoma 

 Mountain, marked 2292 feet high on the map, thence con- 

 tinuing to form most of the mountain ridge which divides 

 that county from Napa, and covering most of Napa County, 

 extend northwest along the boundary between Lake and 

 Mendocino Counties. Though not active in recent times, 



