GEOLOGY OF THE LOWER GILA REGION, ARIZONA. 



189 



Near Osborne's Well there are considerable 

 exposures of sedimentary rocks. Time did 

 not permit a detailed examination of them, 

 but the scattered observations made may be of 

 interest. To the west and south of the well 

 are hills with cliffs cut by the large wash that 

 passes between them. These cliffs expose 

 conglomerate with a calcareous matrix, capped 

 by a basalt flow. The calcareous rock is well 

 bedded. The pebbles in it are in no place very 

 abundant and in the lower portion are lacking 

 altogether. Farther north up this wash are 

 outcrops of red sandstone with concretions, a 

 minor amount of quartz sandstone, and a few 

 small beds of conglomerate. These rocks rest 

 unconformably on gray granitic gneiss. The 

 gneiss, which is very probably pre-Cambrian, is 

 intruded by a light-colored granite, which 

 looks very fresh and is not in the least gneissic 

 in texture. This granite contains a little 

 specidarite and is apparently associated with 

 certain small veins that contain quartz, 

 specularite, and small amounts of sulphides 

 and have been prospected to some extent in 

 this vicinity. The granite, in common svith 

 similar rocks in the region associated with 

 mineral veins, is probably Mesozoic. The 

 sandstone rests unconformably on this granite, 

 as well as on the older gneiss. A distinct 

 thoucrh narrow bed of basal conglomerate con- 

 taining granite pebbles occurs between the 

 granite and the sandstone. A short distance 

 farther north red vesicular basaltic or andcs- 

 itic lava is intcrbcdded with tlie red sandstone. 



E.xposures of sedimentary rocks are found 

 h)V about 8 miles west of Osborne's Well along 

 the road to Parker. There are numerous out- 

 crops of thin-bedded limestone similar in 

 appearance to the matrix of the conglomerate 

 at the well, but entirely free from an}' but very 

 small pebbles. Several of these outcrops are 

 capped with vesiclar olivine basalt. (See 

 PI. XLIII, .1.) They contain rather numerous 

 small and poorly preserved fossils (see next 

 section) and a few small angular fragments of 

 quartz and feldspar. Blanchard '" considers 

 these calcareous beds to be tuffaceous and 

 states that the beds underlying the basalt 

 at Osborne's Well have a groundmass of glass. 



Interbedded with the lavas of Saddle Moun- 

 tain, in Maricopa County, are considerable 

 thicknesses of fragmental rocks ranging from 



le Blanchard, R. C, op. cit.. pp. 24-20. 



agglomerate and breccia of distinctly igneous 

 character to rocks that have angidar fragments 

 of lava about an inch in diameter in a white 

 calcareous matrix. In certain cliffs there are 

 peculiar hollows in beds of conglomerate and 

 agglomerate, some of which almost amoimt to 

 caves. (Sec PL XLIII, B.) The hollows 

 appear to be due to a sort of concave exfolia- 

 tion. They are not the result of solution or 

 erosion. 



Fossih. — The only fossils collected during 

 this investigation were found in the limestone 

 at two localities — in Osborne Wash near Os- 

 borne's Well and in the Clapton Hills. In 

 Saddles Mountain there are beds of calcareous 

 conglomerate which are lithologically very 

 similar to those near Osborne's Well, but no 

 specimens were collected from this locality, and 

 it is therefore impossible to say whether these 

 beds contain any small organic remains such 

 as were found in the limestone near the well. 

 The conglomerate of Saddle Mountain contains 

 small bodies which are, superficially at least, 

 similar to those in the limestone from the two 

 localities mentioned, which W. H. Dall, of the 

 United States Geological Survey, calls '-pseu- 

 domorphs of what were probably a smooth 

 cy pridian crustacean. " 



Specimens from Osborne Wash and from the 

 Clanton Hills were submitted to Mr. Dall for 

 identification of the fossils. A specimen col- 

 lected by John S. Brown, of the United States 

 Geological Survey, from Imperial County, 

 Calif., was submitted at the same time. This 

 specimen, which closely resembles the specimen 

 obtained near Osborne's Well, came from 

 ledge in the west bank of an arroyo at the south 

 entrance to a pass through the Palo Verde 

 Mountains on the road from Glamis to Palo 

 Verde, in either sec. 18 or 19, T. 10 S., R. 27 E., 

 San Bernardino base line and meridian. Mr. 

 Dall states that the specimen obtained near 

 Osborne's Well and the one obtained by Mr. 

 Brown in California "contain the same fossils 

 and were doubtless laid down under practically 

 identical conditions, whether absolutely con- 

 temporaneous or not. " He found in these two 

 specimens 



small oval liodies representing pseu{lomor]ihs of what, 

 were probably a smooth rypridian erustacean, * * * 

 also imprints of fragments of a gastropod which resemble 

 analogous fragments of Mdania, or Goniobasis, and a small 

 triangular bivalve which appears to be most like a minute 



