520 THE MARQUETTE IKON-BEARING DISTRICT. 



which was probably the hast residue of the lava. * * * into these interstices the 

 apatite needles grew, and the alkaline solution attacked the augite and magnetite, 

 turning them into brown hornblende and mica. Finally the heated solution cooled, 

 depositing the quartz and feldspar. * * * 



* * * Olivine, or its alteration product, serpentine, may often be observed 

 microscopically as an occasional accessory, especially in marginal forms, but is seldom 

 abundant enough really to characterize the rock, and certainly is not characteristic of 

 the group as a whole. 



The rock of the knob in sees. 35 and 36, T. 48 N., R. 30 W. (Atlas 

 Sheet VIII), corresponds very closely m the main vpith Lane's description 

 except that no olivine has been detected in the few slides made from it. 

 The aug-ite, moreover, is more or less altered into green amphibole and 

 chlorite, and the cleavage cracks in the plagioclase are lined with the latter 

 mineral. It is also noticed that large crystals of titaniferous magnetite 

 occm- amid the interstitial substances Since this mineral is one of the 

 oldest, if not the oldest, in the rock, its presence among the interstitial 

 substances may indicate that these have not in all cases the origin ascribed 

 to them by Lane. 



Another variety of rock from this knob exhibits a different structure 

 from that described. In a few specimens the augite is in idiomorphic 

 grains and the magnetite in large skeleton crystals. In more altered forms 

 quartz is present as large grains occupying the centers of the interstitial 

 spaces, and surrounding them are beautiful feathery gi-owths of gi-anophyric 

 quartz and feldspar. In other cases the granophyre extends from a nucleus 

 of plagioclase, or forms a zone around plagioclase laths, while again its 

 areas possess the outlines of feldspar crystals. If the granophyre repre- 

 sents an acid base, then the interstices in these specimens occupied a greater 

 volume than did the solid portions at the time the microi:)egmatite began 

 to form. It would seem probable that some of the granophyre at least is 

 secondary. 



OLI VINE-DIABASES. 



Some of the quartz-diabases, as Lane observes, contain olivine in small 

 quantity. There is another class of dike-rocks, however, in which olivine 

 is an important component. These are fresh, heavy, basaltic-looking 

 rocks, which under the microscope appear as very fresh olivine-diabases. 



