GEOLOGY OF THE LOWER GILA REGION, ARIZONA. 



187 



In most places the basalts appear to be the 

 youngest of the flows, for they cap the others 

 and form the summits of the mountains. 

 Everywhere in the region the Tertiary basalts 

 are subordinate in amount to the acidic flows. 

 Thicknesses of 300 feet of basalt are rare, but 

 1,000 feet or more of acidic lava occurs at 

 numerous places. The Tertiary basalts are 

 best developed in the Gila Bend Mountains 

 north of Point of Rocks. 



Interbedded with the acidic flows are beds 

 of siliceous agglomerate and rhyolitic tuff'. 

 The tuff is white or cream-colored, and the beds, 

 which are in places scores of feet thick, are 

 conspicuous. They have a wide distribution 

 throughout the region. 



The flows and tuff's are cut by pipes, dikes, 

 and sills of felsitic igneous rock similar in 

 general composition to the siliceous flows. The 

 quantity of Tertiary intrusive rock exposed is 

 very much less than that of the effusives. No 

 large intrusive masses of this age are known 

 anywhere in the region. A number of plugs or 

 volcanic necks occur in the Plomosa, Dome 

 Rock, and Eagle Tail mountains and some of 

 the other ranges. A conspicuous plug in the 

 Dome Rock Mountains is shown in Plate XLII, 

 A. Court House Rock, a well-known land- 

 mark on the north side of the Eagle Tail 

 Mountains, is a good example of such an in- 

 trusion. It is composed of cream-colored lava, 

 in part weathered to a yellowish brown, and 

 towers 1,000 feet sheer above its base, which 

 is circular and only a few hundred feet in 

 diameter. With the exception of a few cracks, 

 mostly vertical, the walls are smooth and almost 

 vertical nearly to the summit, where the 

 cylindrical column has been partly broken by 

 weathering. This peak is reported to have 

 been scaled, truly a notable feat of mountain 

 climbing. The range itself takes its name from 

 a similar but even higher peak near its east 

 end, whose summit is broken up into three 

 points, showing a fancied resemblance to an 

 eagle's tail sticking straight up into the air. 



About 6 miles west of Osborne's Well, on the 

 north side of one of the outlying hills of the 

 Buckskin Mountains, is a scarp in which a 

 peculiar exposure of igneous rock can be plainly 

 seen. It is shown in Plate XLII, B. This is 

 an intrusion of Tertiary age which differs in 

 several respects from any seen elsewhere in 

 the region. Microscopic examination shows 



that the rock is a gabbro of coarse granulitic 

 texture. The igneous mass has a very irregular 

 outline, and the greatest extension exposed is 

 in a horizontal direction. On the west are beds 

 of bro\vn sandstone dipping about 10° S. and 

 striking rougUy east. The contact with the 

 gabbro is very irregular, and the sedimentary 

 rocks are somewhat baked along it. Directly 

 overlying the igneous rock is a basalt flow which 

 caps the hill and is only 50 feet or so thick. 

 Wlien seen from a distance the lower part of the 

 igneous mass seems to have a rough horizontal 

 stratification, probably due to jointing. The 

 upper part does not exhibit this apparent 

 stratification but weathers in rounded masses 

 2 or 3 feet or more in diameter. The rock in 

 these masses is full of grains of calcite, which 

 give it a pseudoamygdaloidal appearance. The 

 texture differs somewhat from that of the 

 underlying portion, being on the whole coarser. 



This irregular mass of gabbro was clearly 

 intruded into the brown sandstone, which is 

 almost certainly of Tertiary age. The basalt 

 above is probably also Tertiary. There is no 

 evidence to suggest that any other rock covered 

 the basalt at the time the gabbro was intruded 

 below it, but it is somewhat difficult to under- 

 stand how a rock so coarsely crystalline as the 

 gabbro could be intruded within 50 feet of the 

 surface. 



It should be noted that Bancroft " consid- 

 ered all the basalt in this part of Arizona to be 

 Quaternary. Basalts occur on the summits of 

 a number of mountains in the area. The 

 amount of erosion since they were poured out 

 is measured in thousands of feet, so that if 

 these basalts are Pleistocene, some of the most 

 imposing mountain ranges in the area have 

 been produced in large part at least during 

 later Pleistocene or Recent time. At Point 

 of Rocks, on Gila River in the western part of 

 Maricopa County, basalt flows capping un- 

 consolidated gravel of the valley abut agamst 

 the eroded edges of lava mountains. Hence, 

 the basalt flows that cap these mountains must 

 be older than the lava in the valley. As the 

 latter caps unconsolidated gravel it is clearly 

 Quaternary, and it is so greatly dissected by 

 erosion and so much weathered that it is clearly 

 early Pleistocene. From these facts it is 

 evident that the older basalt capping the 

 mountains belongs to the Tertiary. In many 



'> Bancroft, Howlaiid, op. cit., pp. 32-33. 



