330 G. D. LOUDERBACK — STRUCTURE OF THE HUMBOLDT REGION 



trated.* Owing to its direction, the section does not show the tilting of 

 the volcanics, which is mainly to the north. The eastward component 

 of the dip is masked by the deep wave-cut terrace and bold sea cliff, 300 

 feet high, shown in plate 20. 



On the south and west side occur exposures of hornfels imbedded in 

 the granite and broken through by many small aplite dikes. This 

 hornfels is black on a fresh fracture, with a slightly pink tinge. It is 

 very tough, fine, even grained, and rings like an anvil under the ham- 

 mer. On exposure to weathering a thin dark brown film forms on the 

 surface, which, with its other characters, makes it resemble basalt very 

 closely. On account of its peculiar nature and because it has been mis- 

 taken for basalt, and as it is a type of rock abundantly represented 

 along the eastern edge of Trinity mountains, a description of its micro- 

 scopical characters is here given. 



The Lone Mountain hornfels under the microscope shows a very fine 

 and, in general, even grained noncrystalline structure. The chief min- 

 eral constituent is quartz. Careful search failed to give any definite test 

 for orthoclase, and only a single minute grain of plagioclase was seen. 

 The next most abundant mineral is a reddish brown biotite. It has the 

 color of a red garnet in thin-section, and never shows the dark brown or 

 opaque basal sections so common in the biotites of the granites. Its 

 pleochroism is very distinct, from yellow to brown, and in relation to 

 its other properties is, as usual, diagnostically important. This biotite 

 occurs in irregularly shaped anhedrons or in short rod-like sections par- 

 allel to the cleavage. It makes up one-third or more of the rock. Grains 

 of magnetite are sparsely scattered through the mass, and muscovite is 

 present in occasional colorless flakes. A few small, highly refractive 

 grains may be a colorless garnet. The quartz shows frequently a pecu- 

 liar structure. It forms fair-sized aggregates of grains, which extinguish 

 almost exactly simultaneously and occasionally appear to be really a 

 single individual. In the center of such aggregate or large crystal occur 

 biotite and other minerals, while the outer zone is perfectly clear and of 

 quartz only. These outer zones may be wreath-shaped, V-shaped, etcet- 

 era. In the rest of the rock the quartz occurs as irregularly shaped 

 grains mixed quite evenly with the biotite. 



THE HUMBOLDT VALLEY 



From the foot of Lone mountain the Humboldt valley stretches out 

 as an almost perfectly flat floor, except where cut by river or ditch 

 trenches for somewhat over 4 miles. Then the sand dunes begin and 

 the valley rises slowly (100 feet in 2 miles) to the base of the alluvial 



* See page 328. 



