PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 85 



Gabbro.] 



In the magnetite sections that are examined the outlines are not blurred by a 

 leucoxene rim. 



The secondary origin of all the magnetite found in this rock at Duluth, where it 

 is so abundant as to have attracted attention as an iron ore (No. 1C), is therefore more 

 than a possibility; that is, if the term secondary be understood with the above 

 limitation. The fact that the large crystals of magnetite in this rock are seldom or 

 never in their normal, entire, cubic forms, nor even as derivatives from the cubic 

 form, but are of irregular shapes and dimensions, precludes their being considered, 

 as now seen in the rock, as original in a molten magma. Yet, as magnetite, it must 

 have originated early and before the final setting as to crystallization, which the 

 rock now presents. If it be supposed that dynamic or other causes may have 

 destroyed the original cubic shapes, it would still be likely, not only that the other 

 minerals would have suffered similar deformations, but that in some places there would 

 be still some remaining indication of an original quadratic crystallization. Neither 

 of these conditions is found. On the other hand, it is a very common observation to 

 note the encroachment of magnetite upon a grain of pyroxene, and the entire 

 occupancy of the area of the latter by the substance of the former. There the 

 magnetite takes the original shape of the pyroxene, which itself was later than the 

 feldspars, and accommodates itself to its forms. Such microscopic quantities of 

 of magnetite seem to be traceable, step by step, to the larger masses, and the 

 secondary origin of all the magnetite seems to be a reasonable conclusion. It may 

 have resulted from profound changes in the chemical or physical conditions just 

 prior to final congealation, by reason of which some of the elements of the earliest 

 crystallizations were rendered unstable, and a widespread resorption of some of the 

 elements took place, leaving magnetite as a consequence. 



In numerous places in No. 1C, the magnetite can be seen in contact with 

 unchanged augite and feldspar, which shows that it began to form prior to the 

 commencement of decay of the augite. Still, even in No. 1C, it is found in the 

 central portions of what is now a chloritic mass, resulting from change of augite. 

 Here also are some quadratic forms embraced in pure feldspar. 



Chemical analysis of the Gabbro No. 1. The following results were reached by Prof. J. A. Dodge (No. 

 I). Beside it is given (No. II) the composition of a " hornblende gabbro," reported by Streng from the St. Louis 

 river near Duluth. 



I. II. 



SiO 2 50.43 49.15 



A1 2 O 3 23.83 21.90 



Fe 2 O 3 17.05 G.GO (FeO=4.54) 



TiO 2 trace 



CaO 4.79 8.22 



MgO 2.46 3.03 



K 2 O 0.34 1.61 



Na 2 O 2.06 3.a3 



H 2 O 1.92 



