PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 141 



Granite and gabbro. Diabase.] 



Chemical analysis. This analysis was made by Prof. James A. Dodge, and was first published in the 



Thirteenth Annual Report, page 100 (No. 151). 



SiO 2 48.81 



A1 2 O S 23.27 



Fe 2 O 3 11.80 



PeO 3.66 



CaO 5.15 



MgO 1.72 



K a O .75 



Na 2 O 2.38 



H 2 O - . 2.53 



Total 100.07 



Aye. Cabotian. u. s. G. 



No. 46'. RED GRANITE AND GABBRO (contact of). 



Duluth. Weller road, south side of hill. Showing contact of No. IB and No. 1. 



fief. Annual Report, is, page 18. Annual Report, x, page 142. Proceedings American Association for the 

 Advancement of Science, vol. xxx, page 165. 



Meg. The specimens show two rocks in contact. One is a fine grained granite 

 composed of red feldspar, quartz, epidote and hornblende. The other is dark colored 

 and made up almost entirely of plagioclase; in places it contains long thin crystals 

 of this mineral. A few reddish feldspars are also seen in the gabbro. The two 

 rocks are closely welded together and the granite extends in vein-like forms into 

 the gabbro, thus showing it to be at least a little younger than the gabbro. 



Mic. The section shows both rocks. The granite is made up of quartz and red 

 feldspar, with some epidote, hornblende, magnetite and chlorite. The feldspar, like 

 the usual orthoclase or anorthoclase of these red acid rocks of the Cabotian, is much 

 altered and completely filled with minute dust-like red particles (hematite). The 

 gabbro is essentially composed of plagioclase, somewhat altered, and not very 

 distinctly separated into grains and twinning lamellae under polarized light. 

 Throughout this plagioclase and embraced poikilitically by it, are scattered grains 

 and rough rods of magnetite; also chlorite and grains of brownish hornblende. 



The junction of the two rocks is quite a distinct line, and there is no mingling 

 of the elements of one with the other, except that the red stain of the feldspar of 

 the granite has, just at the contact, penetrated to a small extent the feldspar of the 

 gabbro. 



One section. 



Age. Cabotian. u. s. G. 



No. 47. DIABASE (with olivine). 



Duluth. From a dike, same as No. 70, from on outcrop on Superior street. 

 Ref. Bulletin ii, page 110. Annual Report, ix, pages 12, 18. 



Meg. Medium grained, showing a plagioclase, a pyroxene, magnetite, pyrite, 

 and occasionally a spot reddened by hematite; a massive homogeneous rock of the 

 gabbro series. 



