ECONOMICAL GEOLOGY. 179 



Within six miles of the Maricopas wells lies the long granitic chain, which crosses the Gila 

 in a double range, hounding the Jornada ; and from this point westward, along the bed of the 

 river, to Fort Ynraa, there is no locality in which stone cannot be 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 jjurjioses. The white calcareous rock at Tucson is too 

 soft for a building stonoj requiring coherence, but is well adapted for making lime from, and is 

 the most western point known for obtaining that mineral. 



