COMPARISON OF THE HYPOTHESES 309 



Such a fault hypothesis therefore, without any unusual assumption, 

 satisfactorily explains the structural characteristics of the Humboldt 

 Lake range, which may be summed up as follows : 



1. The presence of the basalt (and other volcanics) on the eastward 

 slope of the range for some 12 to 15 miles, the bedding planes forming 

 essentially the topographic slope. 



2. The extension of this basalt from the summit to the east base of 

 the range along which it dips regularly underneath the more recent 

 valley deposits. 



3. The absence of volcanic flows and tuffs on the westward slope of 

 the range. 



4. The occurrence of irregular flat-lying or very low-dipping* patches 

 of basalt along the west base of the range. 



5. The normal longitudinal faulting of the eastward dipping beds. 

 By an anticlinal fold theory it would be very difficult to explain the 



entire absence of the basalt from the west slope, considering its peculiar 

 attitude and extent on the east slope and at the west base, and where the 

 basalt is found at the west base, its lack of any marked westward dip is 

 unaccountable. Furthermore, on the east side we find simple tilting 

 without folding combined with subsidiary normal faulting (as suspected 

 for the valley block), which denotes extension rather than lateral com- 

 pression, the necessary concomitant of folding. 



If this explanation of the structure by fauUing and tilting is true, the 

 west base basalt is part of the same flow which covers the east slope, and 

 the rocks should be petrographically very similar. It is worth while, 

 then, to examine the petrographical details of the rocks from both sides. 



PETROGRAPHICAL DESCRIPTIONS AND COMPARISONS OF SUMMIT AND WEST 



BASE BASALTS 



General characteristics. — In general appearance the rocks look very 

 much alike. The general description has already been given. The 

 upper part of each is rather vesicular, and this character gradually de- 

 creases with depth, showing that in both the surface is near the original 

 flow surface. Crystallization is complete, and a hand specimen shows 

 a mottled appearance due to the alternations of dark and light minerals. 

 No compact black basalt has been found in either of these localities. 



Microscopical physiography of Summit rocJc. — The rock just south of the 

 Muttleberry summit is holocrystalline, with no distinct separation of 

 any of the minerals into two generations, except that a large feldspar is 

 occasionally seen, which may be considered a phenocryst. 



* Flat-lying or low-dipping with regard to an east-west section. One part of the larger area dips 

 south. The smaller area, south of Muttleberry, has a west dip, but not half that of the mountain 

 slope. 



