136 THE MONTHLY BULLETIN. 



GEOLOGY. 



Igneous and nietamorphic rocks have been the main contributing 

 factors entering into the formation of the soils of this belt. In the 

 lower foothills and in local small valleys minor deposits of sedimentary 

 and alluvial material have accumulated and. although of small extent, 

 have an important bearing on the agriculture of this part of the belt. 



SOILS. 



The soils are prevailingly red in color, but this, with depth, texture 

 and composition, varies considerablj^ on account of the complexity of 

 formations and climatic influences entering into their makeup. 



Soil-forming processes have acted under climatic conditions varying 

 from arid to humid. This has had a marked effect upon soils at dif- 

 ferent elevations. The heavy rainfall at the higher elevations has 

 clothed the mountains with a heavy growth of vegetation in most places, 

 which in decomposing has added a higher percentage of humus and a 

 greater water holding capacity to the soils than at lower levels. In 

 places of heavy rainfall moisture has passed downward in crevices, 

 seams and parting planes, causing a deep weathering of rocks in many 

 places. The heavy growth of vegetation in such places has also checked 

 erosion considerably and has given to the soil a deep, rich, red color. 



The soils in lower altitudes are nearly bare in many places, except 

 for a moderate to scant growth of grass. Elsewhere the surface is 

 covered by a moderate to dense growth of chaparral, which affects soil- 

 forming processes but little. These soils are usually low in humus, 

 except on the north and eastern slopes, where the hot penetrating sun- 

 shine has had less effect. The small amount of vegetative covering on 

 the lower foothills has not checked erosion so much, and as a result the 

 soils in many areas have been removed to loAver levels almost as rapidly 

 as they accumulated, leaving a very thin mantle in many places, with 

 varying amounts of rock outcrop. 



Granitic areas generally give rise to light red sandy loams and other 

 loams, but weathering and other agencies have partially removed the 

 iron to the subsoils and have left the surface brown or even gray in 

 places. The other formations give rise to loams and clay loams mainly 

 and the soils are usually much redder, except in the slate areas, than 

 those derived from granitic rocks. 



Small, isolated, irregular areas of dark brown to black adobes occur 

 on the lower slop.es from Stanislaus County southward to the Tehachapi 

 mountains, which, by virtue of their position, fertility and drainage 

 have been found very well adapted to the growing of citrus fruits. 

 The soils vary greatly in depth and are shallowest on the steep slopes, 

 ridges, and where little or no vegetation is present. On the more 

 gentle slopes and where erosion has been cheeked, the soil covering is 

 deeper and ranges from one to six feet. 



In most instances the soil rests upon upturned rocks which permit 

 of the filtering down of soil material to many feet in the seams and 

 .ioints. In such places tree roots were observed developing downward 

 20 or more feet. Wherever shallow soils occur upon highly tilted rocks 

 or where the substratum has been partially weathered, blasting pockets 

 for fruit tree planting has proved very beneficial. The holes formed 

 are filled with surface soil and the creviced or partially disintegrated 



