ECONOMICAL GEOLOGY. 
179 
Within six miles of the Maricopas wells lies the long granitic chain, which crosses the Gila 
in a double range, bounding the Jornada ; and from this point westward, along the bed of the 
river, to Fort Yuma, there is no locality in which stone cannot he obtained within fifteen miles, 
so interspersed is the river valley slope with those isolated hills. The basalt overflow, west of 
the Pimas, affords a capping of hard rock, from 12 to 16 feet thick, in unlimited extent. West 
of this is Goat or Big Horn mountains, yielding granite, quartz rock, and a hard conglomerate; 
and beyond that two isolated ranges of trachyte, porphyry and sandstone, until the hornblendic 
granites of Fort Yuma are reached. The most of the rock met with on the route is igneous, 
either primary or volcanic, both of them affording denser and more durable materials than the 
sedimentary rocks. They are also more exposed and easier obtained. The limestone, where it 
can be obtained, will be preferred for most purposes. The white calcareous rock at Tucson is too 
soft for a building stone, requiring coherence, but is well adapted for making lime from, and is 
the most western point known for obtaining that mineral. 
