GEOLOGY OF THE LOWER GILA REGION, ARIZONA. 



185 



The best exposures noted of the second type 

 of metamorphic rocks of sedimentary origin are 

 those in the Harcuvar, Harquahahx, and Little 

 Harquahala mountains, but they also occur in 

 some of the other ranges. Interbedded lime- 

 stones and quartzites are common, and the con- 

 trast in hardness between the rocks of these two 

 types makes the bedding visible at consider- 

 able distances. The angles of dip in nearly all 

 exposures noted are very moderate, and in 

 most places the beds are nearly flat. Some of 

 the lower beds in this series appear to be similar 

 in composition to the underlying gneissic 

 granite. They are doubtless metamorphosed 

 arkosic sandstones derived from the ancient 

 granitic rock. All the rocks of this series are 

 notably metamorphosed, but many of them 

 are not markedly schistose. Bancroft states 

 that some of the calcareous rocks are dolomites. 

 It is cjuite possible that more detailed work 

 will result in further subdivision of this series. 

 No attempt has been made to measure the 

 thickness, but it is certainly considerably over 

 500 feet. 



These rocks rest unconformably on the 

 granitic gneisses which ^ are so widespread in 

 this region. The best exposure of the uncon- 

 formity found is in the Buckskin Mountains on 

 the road between Midway and Butler Well. 

 At this locality gray granitic gneiss, inclosing 

 masses of sericitic and cliloritic schist, crops 

 out. The schistosity strikes N. 30° W. and 

 dips steeply to the southwest. These rocks are 

 overlain by a mass of distinctly bedded but 

 metamorphosed quartzite and sandstone, with 

 some fine-grained crystalline limestone. The 

 sandstone, especially near the base, is composed 

 of debris from the gneiss below. Schistosity 

 has been developed, especially in the sand- 

 stone, but it is not nearly so pronounced as it 

 is in the lower rocks. The parting planes are 

 parallel to the bedding. The beds have l)een 

 crumpled, but not greatly. The maximum dip 

 observed is 15°, and most of the beds are nearly 

 flat. The average strike is N. 55° W., and the 

 general dip is southwesterly. The section of 

 the mass here exposed is over 150 feet thick. 

 The contact between it and the gneiss and 

 schist is somewhat irregular and is evidently an 

 erosional contact. Both the older and the 

 younger rocks are cut by small dikes of meta- 

 morphosed trap and by a dike composed 

 entirely of microcline. 

 32333°— 22 1.5 



The known occurrences of the least meta- 

 morphosed sediments of the basal complex 

 comprise those at the Socorro mine, in the 

 Harquahala Mountains, and those in the 

 northern portion of the Plomosa Mountains. 

 Bancroft " has studied the section at the 

 Socorro mine and gives the following descrip- 

 tion of it: 



Coarse-grained granite which shows some schistosity ie 

 the basal rock in this locality and is similar to the pre- 

 Cambrian granite so universally present in this area. 

 Resting unconformably upon the granitic rock is a series 

 of slightly metamorphosed sediments, of which about 

 150 feet of fine-grained grayish-red quartzite forms the 

 base. This is overlain by several hundred feet of yellow- 

 ish-brown limestone, the upper portion of which contains 

 intercalated argillites and quartz-mica schists. Strata of 

 schistose shaly limestone and a rotk very closely resem- 

 bling a dolomite (containing, however, fragments of 

 quartz) were noticed near the contact of the quartzite and 

 the overlying limestone. 



In the Plomosa Mountains, near the Little 

 Butte mine, there are limestones similar to 

 those at the Socorro mine, but their relations to 

 the underlying rocks were not determined. 

 At no place were any of these comparatively 

 slightly metamorphosed sedimentary rocks 

 observed in contact with any of the highly 

 metamorphosed sedimentary rocks in the 

 region, and the relations between them were 

 therefore not determined. The lithologic 

 character of the least metamorphosed rocks is 

 similar to that of some of the Paleozoic sedi- 

 mentary rocks at Globe, Ariz. For this reason 

 and because of their relation to rocks that are 

 almost certainly pre-Cambrian and their com- 

 paratively small metamorphism, it seems 

 probable that they are of Paleozoic age. The 

 lack of fossils renders positive correlation 

 impossible. 



There can be little doubt that the granitic 

 gneisses and associated metamorphosed sedi- 

 mentary rocks just described, with the possible 

 exception of the youngest of the sedimentary 

 rocks, are of pre-Cambrian age. The fact that 

 no fossils which can be used to determine the 

 age of the beds have yet been found in any of 

 the rocks examined during the present investi- 

 gation makes all the determinations of the age 

 of formation somewhat uncertain. However, 

 it can not be questioned that these meta- 

 morphic rocks are very old. Some of them 

 might conceivably be Paleozoic, but the 



sBaacroft, Howlund, op. cit., pp. U1-U2. 



