226 THE CRYSTAL FALLS IRON-BE AEING DISTEICT. 



Hornblende. — Tlio liornbleude in the diorites shows a most excellent devel- 

 opment in the prism zone; A-eiy much less well developed are the termi- 

 uating planes. The color varies from dirty green to a reddish-browu. The 

 brown liornblende occupies the center of the crystals, while the green occu- 

 pies the outside, the g-reen agreeing- perfectly, optically, with the brown. A 

 " zonal structvire is indicated by the difference in the character of the horn- 

 blende, though the zones are not sharply delimited, but grade into one 

 anotlier. In a few cases the greenish hornblende grades into one which is 

 almost colorless. The ijleochroism is as follows: Brown hornblende : a, light 

 yellow or light reddish-yellow ; h, light reddish-brown ; jC, darker reddish- 

 brown. Green hornblende: K, light yellow; Ty, bright green; c, dull or 

 olive green. This green hornblende is clearly original and not to be con- 

 sidered as a secondarA' jjroduct after the browii hornblende. Both kinds 

 are free from inclusions. 



Accessory minerals. — Thc epldote is observed very frequently inclosed in the 

 altered biotite, and is surrounded by pleochroic halos. In such cases it is 

 considered a primary constituent. The accessory minerals, apatite, sphene, 

 and zircon, show none other than their usual characters. Titaniferous 

 magnetite is present in the diorites in very small quantity. 



According to the relative proportion of the important minerals just 

 described — plagioclase, orthoclase, quartz, hornblende, and biotite — com- 

 posing the diorites, we get the following varieties: Mica-diorite, quartz- 

 mica-diorite, quartz-diorite, and tonalite. These grade into one another, as 

 stated above; and, as will be shown later, certain of them grade into 

 granites. On account of these variations these dioritic rocks are especially 

 interesting. 



DESCRIPTION OF INTERESTING VARIATIONS. 



SEC. 15, T. 42 N., E. 31 W. 



A dike of rock 4 feet wide, occurring at 425 jjaces N., 1050 W., sec. 

 15, T. 42 N , R. 31 W., near Norway portage, shows the following min- 

 eralogical variation. A sjjecimen taken from the center of the dike shows the 

 rock to be there a typical fine-grained granitite with little or no plagioclase. 

 (Photomicrograph, fig. A, PL XXXIX.) Along the sides the dike rock 

 is a mica-diorite consisting of mica and plagioclase without any quartz. 

 Measurements on zone perpendicular to 010 gave equal extinction angles 



