36 Tue Ottawa Naturalist. 



tion of the olivine. Frequently, however, it occurs in tabular or rod- 

 like forms, which are sometimes arranged in one set of parallel planes 

 only, while in other cases they lie in two sets of planes intersecting one 

 another. These rod-like forms penetrate all the constituents of the 

 rock. In many instances the smaller rod like forms occur in associa- 

 tion with the biotite, and their correspondence in position with the 

 planes of cleavage of this mineral suggests that in these cases, at least, 

 their formation has been due to secondary action (" Schillerization ,: ), in- 

 volving the elimination of the iron and the development of magnetite 

 along the planes of easy cleavage. 



2. Locality. S.E. \, N.YV. , Section 19, 65, 3, cutting on the 

 Port Arthur, Duluth and Western R.R., just west of the narrows of 

 Gunflint Lake, Minnesota. (1) 



Mr. U. S. Grant, who kindly sent me the speciment at Prof. \Y\n 

 chell's request, says : " The rock is from one of the diabase sills (2) in the 

 lower or iron-bearing member of the Animikie. The markedly porphyritic 

 character is only local, the main part of the siil being wiihout phenocrysts. 

 These porphyritic patches are sometimes rather sharply marked off 

 from the main mass of the sill, but they usually pass into the non- 

 porphyritic parts simply by a gradual loss of the large crystals. This 

 sporadic development of large felspar phenocrysts in certain ol these 

 Animikie sills is a rather common feature." 



Macroscopically the rock resembles very closely the boulder 

 brought from lake Temagimi, being a dark green diabase with pheno- 

 crysts of fresh plagioc'ase which exhibit the polysynthetic twin lamel- 

 lation very beautifully. 



The microscope reveals a rock composed mainly of plagioclase and 

 augite with pronounced ophitic structure. The augite when fresh is of 

 the reddish and slightly pleochroic variety so common in diabase, but it 

 shows abundant alteration to greenish or brownish green hornblende 

 (uralite). The opaque iron ore has the same rod-like development 

 noticed in the examination of the preceding rock. Biotite is present 



(1) Specimen No. 951, Geographical ami Natural History Survey ol Minnesota, 

 collectoi 1 . S. Grant, see 22nd Annual Report, p. 82. 



(2) Logan hills ol Lawson, see Bulletin 8, Minnesota Survey. 



