106 THE GEOLOGY OP MINNESOTA. 



[Porphyryte. 



It is evident by the partial darkening of the lamellae that this rock has been 

 subjected to strain and slight distortion. The feldspar in this dike was earlier to 

 solidify than the pyroxene. 



The pyroxene is, in general, much decayed by the formation of the usual greenish 

 chloritic substance, but some of it is entirely free from it. When thus altered the 

 pyroxene is accompanied by the usual appearance of magnetite, which presents a 

 hackly metallic surface in reflected light, and is easily identifiable on turning away 

 the mirror below the stage. There are many areas in the section that were once very 

 probably occupied by olivine, though none of this mineral is now present. These 

 areas are filled largely by sepentine ( ? ) , and the peculiar net-like structure, so common 

 in altered olivines, is well shown. Frequently the dark threads of the net-work 

 have on either side a narrow strip of a rather brightly polarizing mineral. This 

 shows a cleavage running at right angles to the thread, and is distinctly pleochroic, 

 the ray vibrating parallel to the cleavage being greenish, and that at right angles to 

 it straw yellow. The extinction is nearly parallel. A similar mineral is found in 

 small areas elsewhere in the altered olivines; it is probably hornblende. 



One section examined. 



Age. Perhaps Manitou. It is also possible that this is a dike of Cabotian age, 

 which supplied the magma of the supposed sill Nos. 44 ',41 and 43. 



N. H. W AND U. S. G. 



No. 7BC. PORPHYRYTE. 



Duluth. Contact rock between Nos. 7B and 7C. 

 Ref. Annual Report, ix, page 12. 



Meg. A dark gray to purplish, porphyritic rock. The groundmass is very fine- 

 grained, and its components cannot be determined macroscopically. The porphyritic 

 crystals are feldspars of all sizes up to ten millimeters in length ; these are scattered 

 regularly and plentifully throughout the rock. They are light gray to reddish in 

 color, being the latter color more especially where exposed the most to weathering. 

 A little pyrite is visible. 



, 



Mic. The most noticeable feature of the section is the presence of numerous 

 sharply defined, large plagioclase phenocrysts. These are frequently abundantly 

 twinned. Small areas of chlorite and of epidote&re not uncommon in these feldspars. 

 The porphyritic crystals are imbedded in a groundmass of very fine grain, conspic- 

 uous in which are numerous lath-shaped feldspars two- to three-tenths millimeter in 

 length. Besides these the groundmass is composed of a rather confused aggregate of 

 magnetite, green hornblende, chlorite, epidote, minute apatite needles and a weakly polar- 

 izing interstitial substance that may be feldspar or quartz. The chlorite and epidote 

 are secondary, as are also probably the hornblende and part of the magnetite. It is 



