782 PROCEEDINGS OF THE NATIONAL MUSEUM. 



The liibradorite phenocrysts have on M (010) faces an extinction 

 angle of from 20° to 22°, and are consequently somewhat more basic 

 than those of the andesite. 



The amount of feldspar varies considerably in the different speci- 

 mens of this rock. It decreases, apparently, in proi)ortion to an 

 increase in the ferro-magnesian constituents. 



Olivine occurs in beautiful idiomori)hic crystals showing very dis- 

 tinctly the combination of i)ri8m (110) with an acute bracliydome (021) 

 and the brachypinacoid (010). Inclusions of magnetite, augite, and 

 rock glass are fre(]uent. 



The olivine is often colored yellowish brown by limonite, which, with 

 serpentine, form its chief decomposition product. 



Augite, magnetite, and apatite are analogous to those of the andesite. 



Normal olivine-basalt is represented by Nos. 53302, 53303, and 53304. 



The difference between this and the foregoing type lies principally 

 in the holocrystalline development of the groundmass, which contains 

 besides labradorite, augite, and magnetite, a large amount of small, 

 idiom()ri)hi(; olivine crystals (No. 53303). 



The i)orphyritic constituents consist chiefly of olivine with a few 

 scattering crystals of augite, labradorite, and magnetite (No. 53302). 



The olivine of both generations, especially those of the groundmass, 

 are yellowish brown, being strongly impregnated with limonite; from 

 which it would appear that they belong to a ferruginous variety. 



Exogeneous inclusions of slightly fused sandstone are of rather fre- 

 quent occurrence in the scoriaceous andesitic basalt from Mon. No. 15 

 (No. 53297). 



From what has been said it is seen that the rock types encountered 

 by the survey vary from an old, in part highly metamorphosed, granite 

 to the most recent form of basaltic lava. A detailed mapping of the 

 geology of the boundary line on the scale of the topographic map 

 already referred to is much to be desired and would lead undoubtedly 

 to many interesting results concerning the geological relationship of 

 the igneous rock of the southwestern border of the United States. 



