38 



BULLETIN or THE 



blende fragments. But little of tlie(> feldspar shows the twinning of 

 plagioclasc. The structure of the rock renders it probable that it was 

 originally an andeslte. On Light-house point, lying just north of the 

 dike (No. 3, page 3G) first described, and older than it, is an intrusive 

 felsite (quartz porphyry). It is to be here noted that Mr. T. B. Brooks 

 states : '* It may be confidently asserted that no porphyry occurs in the 

 Marquette .... Huronian";* also that this locality has been studied 

 by Messrs. Julien, Credner, Rominger, and others, without finding this 

 felsite. 



This felbite is eruptive generally along the lamination planes of the 

 schist, but at certain points breaks through and across those planes. 

 Figure IG gives a good idea of its relations to the schist and diabase, 

 for such we regard the rock of the dike to be. A short distance to the 

 north, on the opposite side of this immediate spit of land, its eruptive 

 character is better marked than in the first locality. 



The felsito (6) on its weathered surface is colored pinkish and green- 

 ish white, showing fluidal structure, and holding crystals of quartz and 

 pinkish feldspar. On the fresh fracture the groundmass is felsitic, of 

 a greenish-gray color, and holds the same crystals. Pyrite is seen in 

 the fissures. The groundmass is now altered to an aggregate polariz- 

 ing mass, principally of quartz and mica. The fluidal structure is seen 

 in the thin section, and greenish and brownish mica is largely segregated 

 along the fluidal lines. The feldspar is entirely decomposed, having 

 about the same composition and structure as the groundmass, but hold- 

 ing more argillaceous material and less quartz. Chlorite and magnetite 

 wore observed. The original quartz grains are filled with fluid and va- 

 por cavities, and also contain some stone cavities and microlites. North- 

 west of the light-house a more quartzosc felsite (7) was found on the 

 side of a blufl" overhanging a little ravine. This felsite is very much 

 jointed, breaking into small rhomboidal blocks, and cuts through the 

 schists nearly, but not quite, coincident witli their lamination. It is 

 a grayish-white rock containing crystals of feldspar, quartz, and pyrite. 

 Microscopically, the groundmass is now altered to a fine granular ag- 

 gregate, as in the preceding, holding quartz, muscovite, greenish mica, 

 and pyrite. The feldspar is altered the same as the groundmass, and 

 contains similar minerals. The original quartz grains contain micro- 

 lites, stone inclusions, fluid cavities, etc. Wo regard these felsitcs, 

 from their structure, not as the equivalents or precursors of the Ter- 

 tiary rhyoliteSj Ijut as identical with them, the present diflcrence be- 



* GeoL of Wise, III. GGO. 



