564 STUDIES FOR STUDENTS 



the upper sandstones below the lava beds comprise 3,230 feet, nearly 

 all being again vermilion, reddish-brown, or chocolate in color. 

 Occasional partings of a greenish-gray are noted, and the upper 

 shaly beds are characterized by ripple-marks and shrinkage cracks. 

 It would seem from this that if the gray quartzite separating the 

 upper and lower sandstones is, indeed, a beach and marine deposit, 

 that the transgression was but temporary, the beach was pushed 

 back and subaerial deposition continued. 



Following this stage occurred a series of outpourings of lava, 

 thin layers of sandstone separating most of the flows. The lava 

 beds aggregate from 770 to 1,000 feet in thickness, varying this 

 much in a distance of about four miles. The partings of shale 

 and sandstone are widely distributed and of uniform thickness over 

 considerable areas (p. 517) and on Chuar Lava Butte it is noted 

 that the upper lava flow is capped by 35 feet of cholocate-brown 

 sandstone and sandy shales with numerous ripple-marks and mud- 

 cracks occurring among the layers (p. 515). 



A study of the relations of the traps and interbedded sandstones 

 with respect to the alternative hypotheses of marine or continental 

 origin would probably offer some evidence in support of one view 

 as against the other. The widespread character of the lava flows 

 indicates that they were poured out over a level surface. The sub- 

 aerial portion of a delta with a slope of normally not more than a 

 foot per mile is more broadly level than the submerged portion, 

 but, on the other hand, is also more cut by stream channels. 



The more or less viscous nature of lava would cause the upper 

 surface of the flows to depart from a true plane. If above the sea 

 level the streams would tend to erode the upper surface to some 

 extent and result in thicker deposits of sand and clays in the hollows 

 of the upper surface, leveling it once more to grade. These effects 

 might be less marked if the lava flows were poured out beneath 

 the sea. 



In the absence of field study with these points in view it seems 

 best to leave the problem as an open question, but the mud-cracked 

 layers covering the upper lava flow indicate rather strongly the sub- 

 aerial origin of at least that particular sheet. 



Above the last lava flow are found 250 feet of these apparently 



