DIOKITE INTRUSIVBS. 223 



rocks of intermediate acidity, in which the fekispar is plagioclase and the 

 bisihcate constituent is mica or primary hornblende. The feldspar is a lime- 

 soda plagioclase/ 



DISTRIBUTION AND EXPOSURES. 



The distribution of the diorite is limited to a few localities, all of which 

 are in the area underlain by Upper Huronian sedimentaries. The most 

 typical occurrences, and those showing greatest variations, form knobs 

 beginning near Crystal Falls and continuing to the south and south- 

 east. Especially large outcrops form the hills in sees. 19 and 20, T. 43 N., 

 R. 31 W. The smaller occurrences are not indicated on the map. These 

 diorite exposures are always good, so far as getting fairly fresh specimens 

 are concerned, but their contacts with other rocks are almost invariably 

 deeply covered with drift; hence their relations in many cases are doubtful. 



PETROGRAPHICAL CHARACTERS. 



The diorites are holocrystalline rocks of medium to coarse grain. In 

 texture they show some variation from those which are gTanitic to those in 

 which the texture is imperfectl}^ ophitic. The color is, on the whole, 

 moderately light gray or reddish, but at times when the dark minerals 

 become more prominent in the basic facies, especially where we get basic 

 schlieren, the rock is very dark gray or greenish brown. 



The important mineral constituents are feldspar, quartz, biotite, and 

 hornblende. The accessory minerals are epidote, apatite, zircon, sphene, 

 and iron oxides. The secondary minerals, white and brown mica, chlorite, 

 biotite, epidote-zoisite, calcite, and rutile are also present. 



Feldspar. — Plagioclasc feldspar, orthoclase, and microcline occur together. 

 The plagioclase is found in individuals which are fairly automorphic. In 

 the ophitic textured diorites, the plagioclase is the best developed of all the 

 essential constituents. In the granular rocks the degree of automorphism 

 is highest where orthoclase and quartz are present in the largest quantity, 

 and diminishes as these diminish, when the plagioclase individuals begin to 

 interfere with one another's develojDment. For the most part the ^plagioclase 



' Die Eruptivgesteine ties Kristiauiagebietes. I. Die Gesteine dor Grorudit-Tiugiiait-Serie, 

 by W. C. Brijgger, 1894, No. 4, p. 93. II. Die Eruptionsfolge tier triadischen Eruptivgesteine bei 

 Predazzo in Siidtyrol, 1895, No. 7, p. 35. Videnskabsselskabets Skrifter, I Mathematiskuatury. Klasse. 



