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MUSEUM OF COMPAKATIVE ZOOLOGY. 



39 



twccn them being duo to secondaiy alteration, and perhaps their some- 

 what greater depth at the time of eonsolidation than tlie part of modern 



dikes reached by us had. 



On Picnic Point, north of Marquette, a coarsely crystalhne diorito 

 (50, 51, and 52) occurs, forming the main portion of the point. 'J'his 

 rock contains pebbles and fragments (53, 54) in some places. Some of 

 the fragments of schist are large, and one long baud of it was seen. 

 This schist is much indurated, especially near its contact with the dio- 

 rite (58, 59), and forms an irregular junction with it (57). Picnic 

 Islands, just off the point, are composed, in the main, of the samo 



rock (00). 



On the North Island, a diabase dike (01), about twenty-eight feet 

 wide, cuts the *' diorite," running S. 80"" E. Another dike was seen 

 running the same way, on the Middle Island. The "diorite" is some- 

 what brecciated. On Picnic Point a horublcudic granite (55) cuts up 

 through the diabase, and includes numerous fragments of it (5Q). The 

 general structure and relations of the granite to the " diorite," as well 

 as the rounding off of the '' diorito " fragments, are shown in Fig\ire 17. 

 The order is, then, 1st, the schists ; 2d, the " diorite" ; 3d, the granite 



— the question of the priority of either not having been 



and diabase, — 



settled. 



The '' diorite " (50) is a grayish-black rock composed of hornblendo 



crystals, with reddish feldspar, epidote, and pyrite. Microscopically, it 

 contains the same minerals with magnetite and apatite, as well as chlo- 

 rite, quartz, viridito, and other alteration products. The feldspars aro 

 greatly altered, give aggregate polarization, and are fdlcd with altera- 

 tion pi'oducts. 



Tho most coarsely crystalline specimen (52) is a grayish-green rock, 

 composed mostly of short, thick hornblende crystals, showing well- 

 marked cleavage. With tho hornblende a small amount of feldspar 

 occurs. Under tho microscope, besides these, augite, chlorite, quartz, 

 titanito, hematite, actiuolite, and magnetite were seen. The horn- 

 blende and chlorite appear to be products of alteration from the augite, 

 and the hematite from the magnetite. Some of the feldspar can be 

 recognized as plagioclase, and it wovdd scorn that tho rock was origi- 

 nally composed of feldspar, augite, and magnetite, while the other con- 

 stituents aro alteration products. The quartz contains fluid and vapor 



w d * 



cavities. 



The diabase dike rock (01) is a dark gray crystalline one, holding 

 ledge-formed feldspars and weathering brown. In tho thin section it is 





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