226 Longwell — Structure of the Triassic 



most conspicuous constituents. The sandstones are dis- 

 tinctly arkosic, and have a firm cement of calcium car- 

 bonate with some ferric oxide. Throughout the section 

 the predominant color is reddish or pink, although some 

 of the coarser-grained layers are gray. As exposed in 

 the tunnel the beds appear quite regular in thickness, but 

 no doubt in more extensive exposures a lenticular char- 

 acter would be revealed. The only noticeable variation 

 in dip occurs between stations 11 and 12, where the beds 

 are distinctly buckled and are almost horizontal for a 

 few feet. 



The sedimentary beds are cut by numerous joints, but 

 these are not closely spaced except in a few narrow zones. 

 Most of the joints strike northeastward. Only those 

 fractures on which displacement could be detected were 

 studied particularly, and these are indicated in fig. 2. 

 They are sixteen in number, including one in the lava 

 sheet just above the sandstone. No doubt other small 

 faults affect the igneous rock, but in the absence of bedded 

 structure it is difficult to find evidence of displacement. 

 One of the faults represented in fig. 2 was not studied 

 directly, because it is hidden by lining, made necessary 

 by the shattered condition of the rock ; but data concern- 

 ing it were furnished by Mr. Minor, who was present 

 during the construction of the tunnel, and some inferences 

 may be drawn from examination of the closely associated 

 fractures just outside of the concealed zone. The exact 

 strike of this fault is in some doubt. Of the other fifteen, 

 ten lie between N60°E and N75°E (averaging N65°E), 

 and ^.ve between N35°E and N50°E (averaging N45°E). 

 All of the planes (including the one now concealed) dip 

 to the northwest, although one is nearly vertical. The 

 traces shown in the section are in every case flatter than 

 the actual planes, because the tunnel intersects the faults 

 obliquely. The true values of the dips, in order from 

 north to south, are: 66°, 88°, 58°, 70°, 51°, 68°, 65°, 65° 

 (inferred from study of associated fractures, and from 

 Mr. Minor's description), 67°, 70°, 65°, 70°, 62°, 76°, 58°, 

 72°. The average is 67°. 



The normal character of practically all the faults is 

 quite conclusive, and probably this is true of all of them. 

 Definite horizons are difficult to locate in the section, 

 because beds of similar nature and thickness are repeated 

 many times. Observed slickensides are too poorly devel- 



