192 



SHORTER CONTRIBITTIONS TO GENERAL GEOLOGY, 10*21. 



land forms of two general types. These are 

 flat mesas formed by flows that have spread 

 out over the surface of th(^ fill, as at Point of 

 Rocks and Enterprise dam, both on Gila 

 River, and groups of low, more or less conical 

 hills, of which the Bouse Hills, near Bouse, 

 Yuma County, and the Palo Verde Hills, 

 northwest of Arlington, Marico|)a County, may 

 be mentioned as examples. The mesas consist 

 of flows 100 feet thick or less, with a few 

 thicker ones. The hills are in general not over 

 200 or 300 feet high, and many are less than 

 this. The conical shape of many of them 

 suggests that they are volcanic plugs, but all 

 are dissected by erosion, and nowhere was a 

 definite crater found. All the basalt masses, 

 in both mesas and hills, are eroded and have a 

 weathered appearance. The basalt in this 

 area is not nearly as fresh in appearance as 

 much of that in California.-^ The relation of 

 the basalt to the valley fill proves it to be 

 Quaternary, but it is probably not younger 

 than early Pleistocene. 



STRUCTURE. 



Normal faults ai-e the most pronounced 

 structural features of th(> rocks of this region. 

 Thrust faults are not known anywhere in it, 

 anfl only minor fohls appear (o have been 

 fornu'd since pre-Camhrian lime. There seem 

 to have been three general periods of faulting — 



(1) prior to the eruption of the Tertiary lava, 



(2) subseciuent to the eruption of the Tertiary 

 lava, and (3) subsequcuit to the deposition of 

 the older Quaternary alhivium. These periods 

 of movement are not sharply separated from 

 one another. Indeed, it is probable that 

 movement along faidt planers has been in 

 progress almost contiiniously fi-om the begin- 

 ning of pre-Tertiary fauhing to the present 

 chxy. A few of (lu^ mountain rangi^s in the 

 region show no (evidence of being faulted, 

 cither h(M-ause they had a, dilliu'ent. origin or 

 because erosion has entirely removed the evi- 

 dence. 



FOLDS 



The small blocks of early pre-Cambrian 

 seilinientary rocks included in the gneiss in 

 several localities are notably schistose. As 

 regional scliistosity can not be produced with- 

 es Darton, N. H., and others, Guidebook of the western United States, 

 Part C, The Santa Fe Route: U. S. Geol. Survey BuU^ 613, pp. 158 et 

 seq., I'Jl-5. 



out folding, such deformation must have taken 

 place early in pre-Cambrian time. The later 

 pre-Cambrian rocks are in large part not 

 scliistose, and over large areas their strata are 

 flat or dip at gentle angles. Certainly no close 

 folding has taken place in these strata since 

 their deposition. The tipping of the beds in 

 some localities is the result of faidting. As 

 the rocks show evidence of widespread dy- 

 namic metamorphism they must have been 

 subjected to great pressures, which probably 

 resulted in broad and gentle doming. 



In the Eagle Tail Mountains and probably 

 in some of the other ranges the beds of Tertiary 

 lava are curved in a way to suggest gentle 

 local folds. This apparent bending may be 

 and in most places probably is a result of 

 original deposition and not of subse([uent 

 folding. Certainly no large amoimt of folding 

 has affected the Tertiary lavas. 



In the Gila Bend Mountains some of the 

 beds of the older Quaternary alluvium have 

 been gently folded, but most of the Quaternary 

 deposits are undisturbed by folding. 



FAULTS OLDER THAN THE TERTIARY LAVA. 



^Vlien Tertiary volcanism began the siu'face 

 of the I'egion was iiregular. Some of the 

 mountain ranges which are present to-day 

 existed then, although they may not have 

 been as high or as rugged as they now are. 

 The Harquahala and Ilarcuvar mountains 

 and the Ciranite Wash Hills contain no kno%\'n 

 areas of lava and probably never were capped 

 by such material. They are the result of 

 some cause which antedates the lava. No 

 evidence of close folding can be foimd in these 

 ranges. It is possible, even probable, that 

 their uplift was caused by faulting. There are 

 several other ranges in the region that prob- 

 ably belong in this class, but so little is knowTi 

 about them that this is not certain. 



The bold, almost precipitous northwestern 

 face of the Hanjuahala Mt)untains has an 

 appearance that suggests a fault scarp mod- 

 ified by erosion. The abrupt truncation of 

 almost flat beds of pre-Cambrian sedimentary 

 rocks in the southwestern slopes of the Granite 

 Wash Hills near Vicksburg and elsewhere is 

 also suggestive of faulting. Southwest of 

 these hills, only a mile or so from their bases, 

 are small hills of basalt of Tertiary or Qua- 



