GABBEO AND NORITE INTEUSIVES. 243 



HORNBLENDE-aABBRO DIKES. 



One of the exposures of the above-described hornblende-gabbro — the 

 one on the west bank of the Michiganime River in section 29— is very 

 interesting on account of at least two different series of dikes which cut it. 

 The coarse-grained hornblende-gabbro forms the main mass of the knob. 

 The dike rocks may be divided into the fine-grained hornblende-gabbro 

 forming the earliest dikes and the coarse-grained bronzite-norite forming 

 the latest dike. 



Some of the specimens of the fine-grained rock are massive, but the 

 greater portion possess a distinctly parallel texture. These are distinctly 

 micaceous. The rock of the dikes has in general very much the macro- 

 scopical appearance of a biotite-mica-schist. The dikes are very narrow, 

 never more than 18 inches wide. The larger ones send off branches, and 

 in places inclose angular pieces of the coarse diorite. Thus the relation of 

 this fine-grained rock to the ]nain gabbro mass is perfectly clear, though in 

 places it so closely resembles, macroscopically, pieces of mica-schist that in 

 spite of the branching of these dikes, indicating their intrusive nature, they 

 were supposed by one observer to be curiously shaped stringers of the 

 mica-schists included in the main mass of the gabbro. 



The notes do not indicate from just what portions of the dike the 

 specimens were taken; hence it is impossible to state positively that the 

 more schistose parts are nearest the edges and the more granular portions 

 nearer the center, as one would naturally expect. However, in all but one 

 of the specimens which show a contact between the dikes and gabbro 

 which they penetrate, the rock nearest the contact shows a parallel struc- 

 ture. Hence it may be stated that in some cases the edges of the dikes 

 are the more schistose portions. The one specimen refei'red to above is 

 granular and much finer grained along the contact than farther from it. 



The microscope shows the rock of these dikes to be a gabbro differing 

 little in character from the main mass. The plagioclase is well developed 

 in rectangular, more or less lath-shaped crystals. Mica of a rich brown 

 color is rather more abundant than usual, and is in about equal quantity 

 with the hornblende. The hornblende is brown or of a dirty greenish color, 

 containing the inclusions mentioned in the detail descriptions of the minerals 

 of the gabbros. Some irregular grains of a light-green augite (diopside) 

 were observed in the sections. Orthoclase (?) in grains is jiresent simply 



