MUSEUM OF COMPARATIVE ZOOLOGY. 39 



twcen them being due to secondary alteration, and perhaps their some- 

 what greater depth at the time of consolidation than the part of modern 

 dikes reached by us had. 



On Picnic Point, north of Marquette, a coarsely crystalline diorite 

 (50, 51, and 52) occurs, forming the main portion of the point. This 

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

 the fragments of schist are large, and one long band 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 same 

 rock (60). 



On the North Island, a diabase dike (61), 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 hornblendic granite (55) cuts up 

 through the diabase, and includes numerous fragments of it (56). The 

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

 as the rounding off of the " diorite " fragments, are shown in Figure 17. 

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

 and diabase, — the question of the priority of either not having been 

 settled. 



The " diorite " (50) is a gi-ayish-black rock composed of hornblende 

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

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

 rite, quartz, viridite, and other alteration products. The feldspars are 

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

 tion products. 



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

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

 marked cleavage. With the hornblende a small amount of feldspar 

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

 titanite, hematite, actinolite, 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 would seem that the rock was origi- 

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

 stituents are alteration products. The quartz contains fluid and vapor 

 cavities. 



The diabase dike rock (61) is a dark gray crystalline one, holding 

 ledge-formed feldspars and weathering brown. In the thiu section it is 



