MAGMATIC DIFFERENTATION IN ROCKS 15 



head. Go about two miles west, passing over two moraine lines already dissected 

 by transverse valleys, and you overlook an old sandy continuous valley between 

 two moraine lines, now entirely dry, which once drained past West Branch, and, 

 after stopping a while in lake Saginaw (Taylor), passed down the Grand River 

 valley to lake Michigan and the Mississippi. 



The second paper of the afternoon was as follows : 



VOLUME RELATIONS OF ORIGINAL AND SECONDARY MINERALS IN ROCKS 

 BY CHARLKS R. VAN HISE 



Remarks were made by Professor Emerson. 

 The following paper was then read : 



MAGMATIC DIFFERENTIATION IN ROCKS OF THE COPPER-BEARING SERIES 



BY ALFRED C. LANE 



{A bstract] 



In certain of the effusive sheets which so largely make up the Lower Keweena- 

 wan or Copper-bearing series a difference may be noted between the upper and 

 the lower part of one bed or flow other than the difference between the amygda- 

 loid and compact melaphyre. At the top amygdaloid, somewhere about a third of 

 the way below the top of the bed, the rock is like Pumpelly's " ash-bed diabase" 

 type — felsitic, grayish green, with a couchoidal fracture, and conspicuously por- 

 phyritic feldspar. Somewhat above the bottom the rock is of the type of Pum- 

 pelly's " luster- mottle d " type— heavier, darker, not so brittle, showing laths of 

 feldspar embedded in augite patches — that is, ophitic. I have called these two 

 varieties' of melaphyre porphyrite and ophite respectively. There is more augite 

 in the latter type, and observations after Michel-Levy's uiethod on doubly twinned 

 (albite-Karlsbad) sections - of feldspar show a steady, perhaps continuous, variation 

 from AbjAui to AbjAug. 



Analyses A 1 and B 1 are respectively from near the bottom and top of one sheet, 

 A 2 and B 2 from the bottom and middle of another. The silica remains fairly 

 constant, while CaO replaces NajO toward the bottom. The increase in amount of 

 augite (from 15 to 27 per cent)* and in the basicity of the feldspar is balanced by 

 decrease in amount of olivine (from 17 to 6 per cent).* Toward the top the reddish 

 micaceous pseudomorphs of this latter mi ueral are conspicuous, while injthe ophite 

 it is represented only by small corroded granules, crowded in between the augite 

 patches. The estimate of the percentage of constituent minerals originally present 

 in analyses A 1 and B 1 f seems to show that the amount of feldspar remains nearly 

 constant (52 to 55 per cent), so that, as the thin sections indicate, the augite is 

 built at the expense of the olivine. The porphyritic olivine and oligoclase of the 



* Geological Survey of Michigan, vol. vi, pt. i, plate v, and chapter vi. In press. By the kind- 

 ness of the state geologist, Mr L. L. Hubbard, lam permitted to refer to this report, which con- 

 tains much detail in connection with this paper, a little in advance of publication. The part re- 

 ferred to is all printed. 



fLoc. cit., pp. 146, 147. 



