PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 89 



Granite.] 



the structttre as well as some others which will be figured in this volume, but it is 

 still very evident. For illustrations of this class of rocks and this structure, see 

 plate XV of Irvipg's "Copper-Bearing Rocks of Lake Superior," and especially 

 figure 1, which is of a rock from the same locality as the one here described. 



Hornblende occurs in fibrous masses and in compact grains. The fibrous masses 

 occupy areas which were probably originally filled with augite, but the slide now 

 shows no trace of the original pyroxene. A few of these areas show partial outlines 

 that resemble cross sections of augite. The compact hornblende is brownish-green 

 and distinctly pleochroic, a being straw-colored or greenish-yellow, b dark-brownish, 

 and c, nearly the same as b. The absorption is c > b ^> a. The hornblende does not 

 show well defined crystal outlines, but one cross section approaches closely to the 

 form of a pyroxene cross section. The fibrous form is often intimately associated 

 with the compact, the former probably being an alteration of the latter. That the 

 compact hornblende is original, is uncertain, and it is quite probable that all the 

 hornblende is secondary, the original ferro-magnesian constituent of the rock being 

 augite, and the rock one of the augite syenytes described by Irving.* 



Mtif/iK'titc is abundant, occurring in well defined grains, after showing crystal 

 outlines, and in irregular areas and minute grains associated with the hornblende. 

 The rock powder yields many grains to the magnet. 



Apatite is quite common in the form of long needles, which penetrate all the 

 other minerals of the rock, even the magnetite. 



Chemical analysis. The following analysis was made by Prof. J. A. Dodge and published in the thirteenth 

 annual report, page 100 (Chemical series No. 148). 



SiO 2 66.36 



AUO, 13.3:5 



Fe.,O 3 7.89 



PeO 2.1 H! 



CaO 2.14 



MgO 1.20 



K 2 O 3.05 



Na 2 O 2.63 



H,O - 1.21 



100.77 



This shows a rock which is more basic than the average of "red rocks," and which has a lower percentage 

 of soda than is common. 



Remarks. As has already been stated, this rock belongs to the series of " red 

 rocks " of the Cabotian. In Part III, of this volume, will be found a discussion bearing 

 on these rocks; among the various points discussed will be: their origin, their 

 relations to the gabbro, the secondary nature of the micropegmatyte and the 

 hornblende. 



One section examined. 



Age . Cabotian of the Keweenawan. u. s. G. . 



* Op. Oil., pp. 112-124. 



