THE KiNEOUS KOCKS. 521 



Occasioualh' their olivine i.s changed more or less completely into a green, 

 earthv decomposition ])roduct, which extends out from the olivine grains 

 and fills cracks in the neighboring plagioclases, which are usually cpiite 

 fresh. These rocks present no unusual features. They are typical olivine- 

 diabases. The best example of the type is fovind on Green Island, in Lake 

 Michigamme, situated in the SE. \ sec. 27, T. 48 N., R. 30 W. (Atlas 

 Sheet VIII.) 



PORPHYRITES. 



The porphyrites differ from the olivinitic diabases and the quartz- 

 diabases in the absence of quartz and olivine and in the presence cf por- 

 phyritic plagioclases. These rocks resemble the porphyritic phases of the 

 older gi-eeustones, from which, however, they are distinguished by their 

 greater freshness and by the retention of their typical porphyritic structure. 



Although all the porphyrites are, on the whole, so very much fresher 

 than the pre-Clarksburg greenstones, they nevertheless show some altera- 

 tion. Their principal alteration products are those characteristic of weather- 

 ing processes. Decomposition products resulting from dynamic processes 

 are rare, except along local shear zones within the masses of the dikes and 

 along their borders. When much altered the rocks do not differ greatly 

 from the older greenstones in the eastern knobs, except that their original 

 structure is so much better preserved that it can nearly always be observed 

 in the hand specimen. 



A single section made from a specimen collected from a dike cutting 

 the rocks of the iron formation in one of the pits of the Jackson mine, in the 

 SE. i sec. 1, T. 47 N., R. 27 W. (Atlas Sheet XXVIII), is enough different 

 from the types described above to deserve mention. The rock is appar- 

 ently a porphyritic diabase. In the hand specimen it shows white plagio- 

 clase groups in a dark-gray groundmass that is cut by small, almost silky, 

 white fibers. Under the microscope the section shows only fresh pla- 

 gioclase, magnetite, and olivine. The plagioclase is in the usual lath-like 

 forms, which are very small in some portions of the section, and in others 

 are grouped together into large complex aggregates. These latter consti- 

 tute the larger white areas seen in the hand specimen, while the small 

 isolated crystals are the tiny fibers. Many of the feldspars are zonal, with 



