374 /• MORGAN CLEMENTS 



rocks of these chief classes have been obtained and the micro- 

 scopical diagnosis thereby confirmed. Within each of these 

 main divisions well marked varieties can be distinguished. While 

 between the gabbros and the peridotites a transition is unques- 

 tionable, a less positive statement must be made for the connec- 

 tion between the diorites and the gabbros, and while the writer 

 is convinced that such connection exists, he is aware that the 

 reader may not take the same position. In the following pages 

 the three main classes will be briefly described, and then the 

 variations within each class. The relations between the various 

 main divisions will be described, and then, with a brief summary 

 of the facts, the reader will be enabled to draw his own con- 

 clusions. 



Diorites. — Following Brogger's definition, 1 the name diorite is 

 in the following pages restricted to granitic textured plutonic 

 rocks of intermediate acidity, consisting essentially of plagio- 

 clase and either primary hornblende, pyroxene, or mica, or two 

 or more of these. 



This is very different from the usage by a number of the 

 previous writers on the Lake Superior region, who called the urali- 

 tized dolerites " diorites " or " epidiorites." As a result of the 

 introduction of the use of this name diorite by scientific men, 

 it is now in common use among the miners in the Lake Superior 

 iron- regions, and is usually applied by them to any greenish 

 rock whose sedimentary characters are not clearly recognized, 

 very much in the same way that the field geologist uses green- 

 stone, especially if the green rock is associated with an iron 

 formation. In the great majority of cases such rocks are doler- 

 ites in a more or less advanced stage of alteration, and rarely, if 

 at all, pure diorites. 



The dioritic rocks are of medium to coarse grain. In tex- 

 ture they show some variations from rocks of a granitic texture 



J Die Eruptivgesteine des Kristianiagebietes, by W. C. Brogger in Videnskabs- 

 selskabets Skrifter, I Mathematisk- naturv. Klasse. Pt. I, Die Gesteine derGrorudit- 

 Tinguait-Serie, 1894, No. 4, p. 93. Pt. II, Die Eruptionsfolge der triadischen 

 Eruptivgesteine bei Predazzo in Sudtyrol, 1895, No. 7, p. 35. 



