PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 551 



Granite. Diabase.] 



of the common inclusions found in the feldspar of gabbros. The hornblende is green, 

 like that of No. 305, and filled in the same manner, but more abundantly, with 

 apatite. Some of the biotite shows the polarization phenomenon produced by the 

 grinding, as the preceding No. 305, but the bands very irregularly cross one another. 

 The quartz is like that of the preceding. While this rock, in its present stage, makes 

 a good representative of a biotite-hornblende-granite, it is believed to be but a further 

 stage in the alteration process than that shown by No. 305. In both we have the 

 same general characters in their minerals, except that in No. 305 part of the diallage 

 remains, while in No. 801 it has been entirely changed. Macroscopically this rock 

 is like part of the preceding rocks, and this solution of its structure would account 







for the later origin of the quartz and its impression by the other minerals in all 

 so-called granites having the same origin as this. 



"One portion of the rock is darker than the other, and an analysis was made of 

 each portion by professors Dodge and Sidener, with the following results: 



Light Portion. Dark Portion. 



SiO 2 61.19 58.77 



A1 2 O 3 15.22 13.12 



Fe 2 3 3.20 5.45 



FeO 3.55 6.87 



MnO trace trace 



CaO 7.94 5.99 



MgO 2.38 4.93 



Na 2 O 3.17 1.94 



K 2 O 2.62 2.83 



H 2 O 0.40 0.45 



Total - 99.67 100.35 



" These results indicate either that during alteration the basic rocks become 

 more acidic by alteration, as the writer showed* was the case for the peridotytes, or 

 else tha.t this rock is from a more acidic group than the basalts, and that the writer 

 has wrongly placed it with the altered gabbros." 



Three sections. 



Age. Archean (igneous). 



l>i'in//rk. The portions of the rock which are darker colored are simply areas 

 in which the hornblende and biotite are concentrated. The writer would put this 

 rock with the original granites, or the quartz diorytes, rather than with the altered 

 gabbros as does Dr. Wadsworth. u. s. o. 



No. 802. DIABASE. 



From a dike four feet wide, running southwest and northeast, and cutting No. 801. 

 Ref. Annual Report, x, page 107. 



Meg. A fine-grained, heavy, black, diabasic rock. One side of the specimen, 

 which is finer grained than the rest of the specimen, evidently represents the side of 



"IMholorjleal Studies, 1884, pp. 186-189. 



