F. N. Guild — Petrography of the Tucson Mountains. 317 



of ferritic material and occasionally the interior of the crystal 

 has been reabsorbed leaving skeletons filled by the groundmass. 

 (Fig. 3.) The accompanying illustrations (figs. 3, 4) will show 

 the most important variation in this type of basalt. In fig. 4 

 the constituents are more porphyritically dispersed than is 

 usual in these outflows and the groundmass contains more 

 isotropic material, yet to the unaided eye they all appear nearly 

 identical. 



Porphyritic basalt of the first type (a) is found underlying 

 the compact basalt of Sentinel Peak at its southern extremity. 

 That this represents an outflow distinct from the compact 

 variety is shown by the sharp contact between the two, where 

 there is a layer of dark red basaltic tuff and breccia from two 

 to six feet thick. A rock practically identical with this is 

 found at the San Xavier Mission in a small cone-shaped eleva- 

 tion. This variety appears to be made up of large plagioclase 

 crystals constituting nearly one half of the mass, and frequent 

 black lustrous crystals of augite in a groundmass varying from 

 coarse crystalline to compact. The feldspar crystals are some- 

 times over one half inch in length and frequently broken. 

 Under the microscope they are found to be quite fresh, twinned 

 plagioclase filled with dark inclusions of the groundmass. 

 The pyroxene is light yellow with parallel cleavage cracks and 

 high extinction angle. The groundmass, where it can be made 

 out with the microscope, is mostly feldspar and augite with but 

 small amounts of glass. Olivine is not at all abundant and in 

 some slides is absent. (Pig. 5.) 



The second type of porphyritic basalt (5) is found as occa- 

 sional outflows south of Sentinel Peak, the largest yet observed 

 being about seven miles from Tucson. The color of the rock 

 is medium dark gray, and the only minerals which can be 

 determined in it by the naked eye are feldspar and occasionally 

 magnetite. The feldspar is rarely over one quarter inch in 

 length and is more rod-shaped than in the foregoing type. It 

 becomes very conspicuous only as the rock weathers. Under 

 the microscope the feldspar is like that in the first type. The 

 groundmass can hardly be resolved by the microscope but 

 seems to consist mostly of feldspathic material and magnetite. 



The third type of porphyritic basalt (c) occurs in a very 

 small mass not more than one hundred feet in length at the 

 southern base of Sentinel Peak. Portions of the mass are 

 amygdaloidal and much decomposed. The amygdules are 

 sometimes six inches in diameter and filled with agate, usually 

 in concentric rings of varying translucency, or with calcite or 

 siderite. Sometimes there is an outer shell of agate, the 

 interior being filled with calcite. Geodes of brilliant smoky 

 quartz have also been found. In places the rock is sufficiently 



Am. Jour. Sci.— Fourth Series, Vol. XX, No. 118.— October, 1905, 

 22 



