562 STUDIES FOR STUDENTS 



4. Sandstones (lower): 



a) Compact, quartzitic, gray sand rock, 25 feet, with 65 feet of hard, com- 

 pact sandstone 90 



b) Massive, compact, cliff-forming, brown, buff, and purplish-brown sand- 

 stone 1200 



c) I. Reddish-brown to vermilion, friable, shaly sandstone 200 



2. Brick-red, shaly sandstone 250 



3. Brown, friable, shaly sandstone, ripple-marks and shrinkage cracks . . 300 



4. Same in more massive layers, with fine, siliceous conglomerate 80 

 (10 feet) at the base 80 



2120 



5. a) Light gray limestone with interbedded laminas of quartzitic shale ... 8 



b) Brown sandstone with a bed of silicious conglomerate, 2 feet ... 30 



c) Reddish cherty limestone 10 



d) Reddish-brown limestone 2 



e) Dark, reddish-brown slate 5 



/) Light gray, compact shaly limestone 14 



69 



6. Dark, compact basaltic lava in one massive flow 80 



7. Light gray, compact shaly limestone with pinkish tinge between the 

 laminae; it is a little cherty near the base, or with thin, hard, inter- 

 bedded layers of sandstone .... 26 



8. Silicious conglomerate formed largely of pebbles derived from the 

 upturned edges of the pre-Unkar strata, upon which it rests uncon- 

 formably 30 



Total thickness of the Unkar terrane 6830 



Discussion. — The basal silicious conglomerate derived from 

 the upturned edges of the underlying beds and followed by 175 feet of 

 limestones containing a lava bed and some sandstone may be taken 

 as a good indication of the invasion and continued presence of the 

 sea. 



It is noticed, however, that beginning 205 feet above the base 

 are 10 feet of a fine silicious conglomerate which suggests a possible 

 origin as a wave-sorted beach sand. Above this follows 370 feet of 

 brown, friable, shaly sandstones showing ripple-marks and shrinkage 

 cracks. These could be explained as of littoral origin by supposing 

 that the subsidence and sedimentation remained exactly balanced 

 so that this zone was the transition between the subaerial and sub- 

 marine portions of a delta during the entire time of the deposit of the 

 370 feet. Even assuming a littoral origin, however, the immediate 

 vicinity of a shore is implied and it seems a much simpler hypothesis 

 to suppose that the mud-cracks were of flood-plain origin. Under 

 this assumption the cracks could be more readily accounted for 

 and it is not necessary to postulate an exact balance between the 



