1894. 



PROCEEDINGS OF TEE NATIONAL MUSEUM. 



645 



merely suggestive. The following results were obtained by Mr. Eakins 

 on a samj)le iu which the augitic alteration thus described was com- 

 plete : 



SiOj . 

 TiO, . 

 Al,03 

 Cr,()3 

 Fe,03 

 FeC) . 

 MnO. 

 liaO . 



Percent. 



49.47 



.21 



12.15 



Trace. 



1.93 



4.07 



.10 



.03 



CaO . 

 MgO. 

 KjO , 

 Ka^O 

 HjO , 



P,05. 



COj.. 



Percent. 



9.30 

 10.86 

 2.42 

 2.08 

 4.14 

 .'il 

 3.31 



Making due allowances for the various changes attending decom- 

 position, the rock, it will be noted, agrees closely with that from Cot- 

 tonwood Canon (No. 38596, U.S.N.M.) and Boulder Creek (Xo. G2409, 

 U.S.:N^.M.), to be noted later (p. 070). 



The association of this rock with that next to be described is pecu- 

 liar and needs' farther investigation. 



Mica syenite. — Underlying the above. In the hand si)eciinen the 

 rock (Ko. 38600, U. S. I^. M.) appears to the unaided eye as a holocrys- 

 talline mass of pink lath shaped feldspars interspersed with very 

 numerous long, slender, and at times radiating needle-like folia of black 

 mica. As seen under tlie microscope the structure is quite simple, con- 

 sisting of a holocrystalline aggregate of badly kaolinized sanidins, lath- 

 shaped plagioclases, scales of mica, scattering granules of iron oxide, 

 apatite needles, and in the interspaces, secondary calcite, plagioclase, 

 and, rarely, quartz. 



The most interesting feature of the rock is the almost constant inter- 

 growth of the sanidins with plagioclase, the effect being in the section 

 as if each crystal of plagioclase was set in a frame of orthoclase, 

 as already described and figured in Bulletin 110, U. S. Geological 

 Survey. Unfortunately, in the sample at hand, both feldspars are so 

 badly decomposed that their optical properties are greatly obscured. 

 The plagioclase alteration gives rise to innumerable minute flecks of a 

 silvery white micaceous mineral, and in many cases the twin structures 

 have become entirely obliterated ; the potash feldspar has become brown 

 muddy, and opaque, resembling the orthoclase of granitic rocks, and at 

 times acts scarcely at all upon polarized light. A partial analysis of 

 as fresh a sample of the rock as could be obtained, yielded Dr. Chatard 

 results as follows: Silica, 58.88 per cent; pota.sh, 5.18 per cent; soda 

 3.46 per cent. The rock is undoubtedly a phase of the syeuitic 1am- 

 prophyre, which was later found in the vicinity of Antelope Creek 

 (p. 671). 



Porphyriie. — Intrusive sheets between Dry Creek and Nixons Basin. 

 The rock here is evidently identical with that of the lower sheet of 

 Cottonwood Creek, though the sample collected by Dr. Peale is so 

 badly decomposed that little can be made from it. 



