206 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



In summary we have then as original constituents, labradorite, 

 augite, magnetite, pleonaste, apatite, biotite, and oHvine ; as secondary 

 constituents, hydrous iron oxide, chlorite and perhaps a small amount 

 of serpentine. Of these original constituents the labradorite and 

 augite only rank as essentials, the remaining constituents are 

 accessory. The rock, therefore, may be classed as an olivine gabbro. 



Quartz Monzonite. — This rock (Cat. No. 75,491), of which a 

 single specimen was secured, is light gray in color with a ground 

 mass composed of feldspar with semi-lustrous cleavage plates. 

 Scattered through this white ground mass lie elongated crystals of 

 a black mineral, hornblende. In some places, locally segregated, 

 occur bunches of mica flakes. Drusy cavities are quite frequent. 

 An occasional pellucid quartz was also noted, but though macro- 

 scopically evident, it is a very minor constituent. 



Though apparently fresh, the rock, a surface sample, has under- 

 gone considerable alteration and weathering, and the feldspars are 

 considerably kaolinized, presenting under the microscope grayish- 

 brown cloud-like masses, which, however, give the dull-gray inter- 

 ference tints. 



In texture the rock is panautomorphic ; all the grains are crystal- 

 line, approximately the same size, approaching automorphic forms, 

 yet few or none possessing them. 



In thin section the appearance of the rock duplicates its m.ega- 

 scopic features. By far the most abundant constituent is feldspar. 

 This has decomposed to such an extent that in some spots identifica- 

 tion is impossible, and even when freshest its identification is obscured 

 from the large quantity of separated material, which makes the 

 cleavage lines most indistinct. This separated material is, for the 

 most part, kaolin. It is brownish in color and is made up of flakes 

 and needles without any action on polarized light. It is scattered 

 in translucent clouds throughout the mass of the feldspar. This 

 latter mineral gives in some sections parallel extinctions ; in others, 

 the angles range from 8° to 18° ; such sections evidently approach 

 the clinopinacoid and show an elongation of the feldspar parallel 

 to the c axis. Though generally untwinned, in places albite lamella 

 were noted. These small extinctions point to the presence of albite 

 and oligoclase. In some spots, in the neighborhood of larger 

 quartzes, a micropegmatitic development of quartz and oligoclase 

 was seen. Carlsbad twinning was also noted. 



Additional to these acid feldspars, another group of plagioclases 

 is present of a much more basic character. These basic feldspars 

 are twinned both according to the albite and Carlsbad laws, hence 



