PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 137 



Diabase.] 



No. 43. DIABASE. 



Duluth. On the hill slope, back from the base of Minnesota point, in front of the engine house. Width 

 and form of thisjock could not be made out. Surface rounded over by glaciation. Apparently has a dip east 

 30 north. Approximately the equivalent of No. 6A, and of No. 44 ', and perhaps of No. 41. 



Ref. Annual Report, ix, pages 12, 17. Annual Report, x, pages 107, 109. 



Meg. Generally of very uniform character, but in some places finely porphyritic 

 with a red feldspar. 



Mic. The feldspar, without further specification, may be said to be a plagioclase, 

 and probably labradorite. It is much changed, showing secondary microlites of 

 various sorts. 



The pyroxene is also much changed, some of it being entirely lost, but some still 

 plainly preserving sufficient of its molecular structure to give the high polarization 

 colors characteristic of thick sections. 



Olivine grains are quite common, but largely serpentinized so as to give an 

 aggregate polarization. They are the favorite gathering-places for magnetite which 

 forms strings and reticulations marking the original irregular cleavages of the olivine. 

 It is apparent that in some cases olivine grains have been entirely replaced by mag- 

 netite. These olivine grains preceded the feldspars in generation, although they do 

 not now manifest perfect crystalline outlines. 



A considerable portion of the section is occupied by a mineral substance now 

 changed so as to give a similar aggregate polarization, but coarser. This is appar- 

 ently the same that was described in section No. 41 as uncrystallized magma. Toward 

 this the feldspars are idiomorphic but it also spreads widely throughout the slide. 



Magnetite is common, and apparently is entirely a secondary product, occupying 

 the place of either olivine or of pyroxene. 



Chlorite scales are sparse in the changed feldspar, but more common as a product 

 of decay of the pyroxene and in the masses of undifferentiated magma. 



Pyrite in fine grains is seen occasionally. 



Age. Cabotian, allied to the Beaver Bay diabase. N. H. w. 



No. 43 A. DIABASE (with olivine). 



Duluth. Same locality as No. 43. 

 Ref. Annual Report, ix, page 17. 



Meg. A fine-grained diabase with small, red, porphyritic feldspars. The red 

 color also penetrates somewhat into the mass of the rock. 



Mic. The rock is essentially like No. 43, of which it is a part. However, most 

 all of the feldspar is reddish. The porphyritic feldspars are very highly altered, but 

 some of them still show traces of polysynthetic twinning. 



One section. 



Age. Cabotian. u. s. G. 



