212 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



ophitic texture to the pilotaxitic texture. The feldspars in it are in small lath- 

 shaped individuals, and, like the phenocrysts, are fresh. The augite of the 

 groundmass is to a g-reat extent altered to uralite, and the ii'on ores to sphene. 

 One of the dikes is about 5 feet wide. In the center it is a moderately 

 fine-g'rained rock; on the edges it is a dense aphanitic basalt. Even in thin 

 section the g-radation from the rock with microophitic groundmass to the 

 one with a dense pilotaxitic groundmass is well shown. A dike of larger 

 size might readily have cooled sufficiently slowly to have crystallized at its 

 center as a dolerite. 



UliTRA-BASIC 13VTRUSIVES. 



Under this head are the descriptions of the picrite-porphyries (porphy- 

 ritic limburgites). 



PICRITE-PORPHYRY (PORPHYRITIC LIMBURGITE). 

 GEOGRAPHICAL DISTRIBUTION AND EXPOSURES. 



The picrite-porphyries occur in isolated outcrops of comparatively 

 small size in sees. 9, 22, and 27, T. 44 N., R 32 W., in. the area supposed 

 to be underlain by the Lower Huroniau Hemlock volcanics. They are 

 surrounded by outcrops of the altered poikilitic dolerites, but the exposures 

 are not sucli as to allow their relations to be determined. Their occurrence 

 jDoints to an intrusive character. It is on account of their field occur- 

 rence alone that we feel justified in describing them here under the general 

 heading for this chapter, "Intrusives," instead of under the volcanics with 

 the basalts, their proper place from a strict petrographical standpoint. 



PETROGRAPHICAL CHARACTERS. 



The -picrite-porphyries are medium-grained rocks, which vary in color 

 from gray to dark green and almost black. In general they have a por- 

 phyritic character. This is, however, not so well marked in the gray as in 

 the darker-colored rocks. The gray ones have a spotted appearance. The 

 spots are gray in color, fibrous, very rarely larger than 3 or 4 millimeters 

 in length, and lie in a finely fibrous, dark-green matrix. In the dark rocks 

 the porphyritijc crystals reach a length of 1 centimeter, and are bluish to 

 black, with silky luster. They lie in a fine-grained, more or less fibrous, 

 green groundmass. In one of the dark rocks the magnetite is very notice- 

 able. The crystals project from the weathered surface and the rock is 

 strongly polar-magnetic. 



