Calkins.] 



Petrography of the John Boy Basin . 



123 



from various flows, viewed maeroscopically, exhibits much vari- 

 ation in structure and color, but a microscopical study proves that 

 these variations are due to differences in conditions of consolida- 

 tion and degree of weathering, while the rocks are mineralog- 

 ically, and no doubt chemically, of the same type. These 

 macroscopical variations may be illustrated by describing three 

 typical hand-specimens. 



A specimen collected near the foot of the canon has a compact 

 gray or drab ground-mass, in which are abundant small dull white 

 crystals of striated feldspar, small prisms of black pyroxene, and 

 grains of a soft olive-green mineral. A second specimen, from 

 farther up the canon, has a ground-mass of olive-green color, 

 with phenocrysts of notably large size. The most prominent are 

 of feldspar, always distinctly striated, and strongly flattened on 

 the brachypinaeoid. These often attain dimensions of about 

 2 mm. x 8 mm. x 10 mm. Of smaller size and prominence are 

 the stout prisms of greenish black pyroxene. The soft olive-green 

 or brownish green substance observed in the first rock occurs here 

 in prisms of very well developed crystal form. Its nature will be 

 discussed in connection with the microscopical characters. A third 

 specimen, from near the head of the canon, is a dark gray rock 

 having somewhat the appearance of a basalt. The abundant 

 small phenocrysts of striated feldspar are generally glassy, and do 

 not stand out prominently from the ground-mass, but they are 

 sometimes stained a bright red by iron oxide. On close exam- 

 iion, small prisms of pyroxene may be detected, often asso- 

 ciated with a soft greenish substance. 



The last-described specimen, being the freshest of the three, 

 may serve as a type for microscopical study. Its microscopical 

 characters will therefore be described at some length, and special 

 features observed in other specimens will afterward be noted. 



In thin section, it is seen that the phenocrysts are plagioelase 

 and rhombic and monoelinic pyroxene, the two latter minerals 

 being sometimes intergrown. Magnetite also occurs in two 

 generations. All these minerals occur in a holocrystalline 

 groundmass, cemented with a paste of alkali feldspar. Apatite is 

 an accessory. Decomposition has affected the hypersthene, and to 

 a smaller extent, the phenocrystic feldspar.* 



* See plate 11, tig. 2. 



