THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 77 



closely to the Great Valley, so that roughly from the latitude of 

 Sacramento northward the climax chaparral is excluded from the 

 mountains and foothills proper, such scrub species as occur being 

 of the conifer-forest group. It is an interesting fact that in this 

 stretch of the Sierras Adenostoma is almost absent. North of Auburn 

 I have failed to find it in four crossings of the foothills and have 

 been able to discover but one record of its occurrence, noted by Dr. 

 Jepson northeast of Chico. 



The relations between chaparral and grassland present certain 

 problems difficult and perhaps impossible of full solution. The 

 fact of the greater extension of the chaparral in the past is certain; 

 the magnitude of that extension is the point of difficulty. The 

 very general statement may be made, subject to qualifications 

 about to be indicated, that in central California the lower mountains 

 are controlled by chaparral and the plains by grasses. The character 

 of the transition zone between the two types is as follows: 



The first hills are as a rule entirely grass-covered, though even 

 on these, and occasionally out upon the valley-floor, are patches 

 of chaparral. These show absolutely no correlation with altitude, 

 slope-exposure, or soil-type. Their edges are sharp and the shrubs 

 are uniformly developed throughout. They are obviously remnants. 

 Penetrating farther into the mountain mass, the chaparral patches 

 become more and more numerous, but are still arranged purely in 

 hit-and-miss fashion. Farther still, the chaparral controls the 

 greater area, and the grassland forms the patches, which in summer 

 appear like tan-colored inlays set into the green of the scrub. Finally, 

 the grassland disappears, leaving the chaparral in complete control. 

 In such a journey we do not necessarily encounter continually higher 

 altitudes; we are merely penetrating more and more deeply into a 

 mountain complex in which the ridges may all be of similar height. 

 In short, everywhere near the valleys and plains the hills are grassy, 

 while in the depths of the ranges they are covered with scrub. The 

 larger the extent of the mountain mass the greater is the central 

 area of chaparral. Conversely, a small isolated area of hills, though 

 of considerable altitude, may have none. This arrangement is so 

 nearly universal where chaparral and grassland meet that specific 

 examples are hardly necessary, and yet perhaps a graduated series will 

 be of interest. 



The Marysville Buttes in the Sacramento Valley are excellent 

 examples of the isolated hill with practically no brush. The highest 

 summit is 630 meters and the slopes are everywhere covered with 

 the typical annual vegetation of the plains. Even here, however, 

 there are suggestions ol the chaparral in the presence in small numbers 

 of Quercus dumosa, Q. wislizeni, and Heteromeles arbutifolia (44). 



Mount Diablo is also an isolated mass, but of greater extent and 

 height than the Marysville Buttes. Its lower flanks and the sur- 



