1906 



FORESTRY AND IRRIGATION 



225 



pronounced irregularities creep in, 

 however, the determination of age by 

 the counting of rings becomes a task 

 of great difficulty. With the decidu- 

 ous species the case is different. They 

 usually present plainly but one annual 

 ring per year. 



The geologic structure of this drain- 

 age basin is varied and interesting. In 

 the southeast portion the gentle swells 

 are paved with great blocks of car- 

 boniferous limestone. Resting upon 



formation from northeast to southeast, 

 and together with erosion, govern the 

 minor topography. These audesite 

 dikes are often sufficiently mineral- 

 bearing to deflect a transit needle sev- 

 eral degrees from its true course. As- 

 cending nothward, we find resting 

 upon this formation a body of rhyo- 

 lite, a whitish acid, volcanic rock, its 

 edge exposed in the form of several 

 water-sculptured terraces that repre- 

 sent as many successive flows of lava. 



Photo by \V. R. Matoon. 



A Closer View of North Slope. Note the deltoid mass of rock fragments and debris 

 in left loreground at base of slope. Immediately above the gully is ten feet 

 deep. This slope not long ago was covered by an open stand of Bull Pine. 



this are certain bodies of quartzite of 

 perhaps Triassic age. Mt. Humboldt, 

 marking the easternmost point of the 

 basin, is a cone built of this exceed- 

 ingly hard material. All the remain- 

 ing foothill portion of the watershed 

 belongs to an eruptive formation, prob- 

 ably Older Tertiary. It contains cer- 

 tain bodies of volcanic sandstone. Ribs 

 of intrusive rock, usually 8-12 feet 

 wide, at intervals traverse this whole 



Surmounting this, and marking the 

 line where the foothills merge into the 

 mountain slopes, come bodies of con- 

 glomerate hundreds of feet thick. 

 These are composed of volcanic mate- 

 rial, but evidently deposited by water. 

 Spread upon top of the conglomerate, 

 and showing itself continuous through- 

 out the watershed and beyond, is a 

 layer of brown, Miocene or Pliocene 

 basalt, whose edge forms the perpen- 



