PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 315 



Quartz.] 



Remarks. No careful study of the feldspar of the great Saganaga granite has 

 yet been made. This mineral is usually too much altered for a satisfactory optical 

 examination. It is, however, clear, from the sections examined, that a considerable 

 part of the feldspar is plagioclase. This shows fine twinning lamellae according to 

 the albite law; in grains showing an equal extinction angle on each side of the 

 twinning line the extinction angle is very low, indicating that the feldspar is most 

 probably oligoclase. 



An analysis of a characteristic specimen (made by Mr . A. D. Meeds) of the Saganaga granite from the 

 S. W. % N. E. # sec. 22, T. 66-5 W. [see Annual Report, xx, page 88; Annual Report, xxi, page 43, No. 086 (G.) ] 

 is as follows : 



SiO 2 69.34 



A1 2 O 3 17.25 



Fe '' ' 046 



PeO <, 



CaO 3.43 



MgO 1.18 



K 2 .71 



Na 2 O 4.33 



H 2 - 1.17 



Total - 99.87 



From the analysis it is seen that very little potash feldspar is present, and in 

 this case the rock might properly be called a quartz dioryte. It is possible that 

 other samples of this rock from different parts of the lake would show a much 

 higher percentage of K 2 0; at any rate, on account of the large quantities of quartz 

 and the general character of the rock as a whole, it seems best to refer the great 

 Saganaga mass of coarsely crystalline quartz-feldspar-hornblende (or biotite) rock 

 to the granites rather than to the diorytes. u. s. G. 



No. 318. QUARTZ. ( Vein matter. ) 



Northeast side of a small island in Saganaga lake; S. E. J| N. W. % sec. 14, T. 66-5 W. 



Ref. Annual Report, ix, page 84. (Compare No. 835W.) Annual Report, xvi, page 215. (For a descrip- 

 tion of this island, on which is a fluorite granite, see A. WINCHELL, Annual Report, xvi, pages 215, 216; U. S. GRANT, 

 Annual Report, xx, page 89; N. H. WINCHELL, Annual Report, xxiv, page 22, No. 2046.) 



Mc(j. Coarsely crystalline, pure, milk-white quartz. There is also a very little 

 purple fluorite. 



M'/i'. The section shows a large mass of clouded quartz, with similar orientation 

 throughout. The cloudiness is due to innumerable minute cavities filled with liquid, 

 and frequently containing a bubble. 



One section. 



Age. A vein in Archean rocks. 



Remarks. From the field descriptions, reference to which is made above, it 

 appears that this island contains an irregular quartz vein. The surrounding rock, 

 which is the usual Saganaga granite, has been much decayed and silicified, and in 

 some places penetrated with purple fluorite, forming a beautiful fluorite granite. It 



