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MUSEUM OF COMPARATIVE ZOOLOGY. 



87 



48, 49). A specimen of one of these (45), taken near the edge of the 

 dike, is composed of plagioclase, orthoclase, magnetite, titaniferous 

 iron, hornblende, viridite, quartz, epidotc, and augito. The augite is 

 generally nearly altered to hornblende and viridite, only a little of 

 the distinguishable augite remaining in the centre of the crystals. 

 The other constituents, except part of the feldspar and the iron, boar 

 the characters of alteration products. A section adjacent to the 

 junction with the schist shows much higlicr alteration. The augite 

 has disappeared, the iron has been nearly all altered to " Icucoxene," 

 and secondary orthoclase is abundant. The schist from the same speci- 

 men (45) is coniposod of quartz, argillaceous material, chlorite, horn- 

 blende, magnetite, ^'leucoxcne,'' and a little augite. It would seem that 

 this has been formed from detrital material of the same nature as the 

 dikes (basaltic). The close resemblance of the '' diorite " and schist in 

 mineralogical characters, but not in structure, is shown in another sec- 

 tion containing the junction of the two rocks (48). No. 49, taken from 

 the centre of the dike, sixty-six feet wide, running parallel with and 

 belonging to the same system as the others, is composed of plagioclasc, 

 augite, magnetite, olivine, and some viridite. Long microlites of apa- 

 tite traverse the mass in various places. The feldspar contains numer- 

 ous inclusions of the original base, more or less altered, of the same 

 general structure, relations, and arrangement as is commonly seen in 

 the feldspars of modern basalts. The portions of the base originally 

 left in the crystallization of the molten magma in the interstices between 

 the crystals has been changed to a grayish or brownish fibrous sub- 

 stance, also to viridite. The augite is comparatively fresh, and cut by 

 the feldspar crystals. The structvu-e of the rock is that belonging to 

 the more coarsely crystalline basalts. The altered base is probably the 

 ''inserted substance" of Drs. Zirkel nnd Wichmann.* It would seem 

 that some of the above rocks arc like the " diorite " of Dr. Wichmaun 

 from this locality (/. c, p. 629). Wc feel that the lithologist who was 

 unacquainted with the field relations of the preceding specimens would 

 declare it impossible that they covild be from the same series, apparently 

 identical in age, and originally so in composition. South of Marquette 

 the schists (11) are more argillaceous, forming true argillites, although 

 chloritic in places. Numerous dikes were seen here, but the rock is 

 more altered, and perhaps was extravasatcd earlier than most of the 

 dikes on Light-house point (9, 10, 12). No. 12 is composed of f\^ld- 

 . spar, quartz, viridite, opacite, and pseudomorphous remains of liorn- 



* GeoL of Wise, III. 62i. 



