424 



University of California. 



[Vol. 3. 



THE ROCKS OF THE REGION. 



Structurally, the region about Orleans is characterized by two 

 eastwardly tilted fault blocks. They are separated by one of 

 the great thrust faults of the Klamath Mountains. It occurs less 

 than two miles east of Orleans and has a course from north to 

 south. The throw is at least one mile and probably two, and the 

 hade seems to be toward the east at a rather high angle. The 

 formations on opposite sides of this fault are totally different. 

 On the east there are mainly Paleozoic cherts and slates intruded 

 by vast quantities of igneous rock. There seem also to be a few 

 limited areas of more ancient schists, and a few limestone lentils 

 have been noted. The igneous rocks play a more important role 

 than do the stratified rocks, especially along the Klamath River. 

 They occur chiefly in immense irregular masses rising vertically 

 or nearly vertically through the Paleozoic strata. They occupy 

 much larger areas than the latter. 



On the west of the fault there is a long north-south belt of 

 Bragdon slates and crushed sandstones. It is from three to five 

 miles in width and is not interrupted by igneous rocks with the. 

 exception of a few serpentine and granite areas. It is underlaid 

 by igneous rocks which have been sheared and altered to sericitic 

 and other schists. They rise along the west border in a narrow 

 north-south belt and are bordered on the west by another fault, 

 beyond which occurs another Bragdon area. 



All the rocks of the region are resistant to erosion, but the 

 Bragdon slates, by reason of their sheared condition, are much 

 less resistant than are the Paleozoic strata and igneous rocks. 

 This is an important factor in the control of the topography. 

 The mountain summits of the Bragdon belt are lower than those 

 on either hand, and the valleys are broader, especially than those 

 excavated in the igneous rocks. 



BRIEF OUTLINE OF GEOMORPHOGENY. 



The higher summits of the northeastern portion of Humboldt 

 County represent the Klamath peneplain.* The finest remnant 

 is the flat-topped "Trinity Summit," 13 miles south of Orleans. 



* Topographic Development of the Klamath Mountains. U. S. Geol. 

 Sur., Bull. No. 196, pp. 15-18. 



