BUILDING STONES. 147 



Crystalline rocks. 1 



this rock was opened by Messrs. Saulpaugh Brothers in 1881, for use in the 

 Northern Pacific railroad bridge at Bismarck over the Missouri river. 



Microscopic characters of No. 9. The feldspar of this rock shows the twinning striation of 

 plagioclase in some of its grains, but it is wanting in a large portion of them. Magnetite accom- 

 panies the biotite, and slender cylindrical colorless microlites cut through the feldspar. Pyrite in 

 small quantity is associated with hornblende. 



The so-called "granite" of Duluth (No. 2), quarried at Rice's Point, 

 belongs to a very different class of rocks, and is now generally designated 

 gabbro by lithologists. This term, derived from Italy, is applied to an igne- 

 ous rock consisting of the triclinic feldspar labradorite, augite and magnet- 

 ite. These minerals are all softer than quartz, which is wholly absent from 

 the Duluth rock, but which makes up so large a part of the foregoing syen- 

 ites. The rock, however, is more difficult to quarry on account of its tough- 

 ness and homogeneity. It has no gneissoid structure, and the cleavable 

 labradorite has but little effect in producing an easier fracture in one direc- 

 tion than in another. While, therefore, taken in mass, this rock is softer 

 than the St. Cloud syenites, it is more difficult and expensive to quarry and 

 to reduce to convenient blocks. 



This gabbro makes the chief rock of an important range of hills in Min- 

 nesota. The " Mesabi " in much of its extent consists of the same rock. It 

 is found to vary somewhat in its color and composition, yet always within 

 narrow limits, constituting on the one hand the "felspar rock" of Norwood,* 

 where the ieldspathic ingredient predominates largely over the other min- 

 erals, and is of a clear, almost glassy transparency, and of a gray color, 

 weathering nearly white, and on the other hand the "trap-rock" so-called, 

 as it is displayed at many interesting points along the shore of lake Supe- 

 rior, where it has frequently been described as "greenstone." The green 

 color in the latter case results from the change of the augite to delessite, or 

 to* some chlorite-like mineral under the influence of the weather, and from 

 the absorption of iron. The former variation from the typical gabbro is 

 No. 8 of the general table, and the latter is No. 11 or No. 3 ; the last being 

 from Taylor's Falls. 



Microscopic characters of Nos. 2 and 8. The labradorite which composes the largest part of 

 this rock exhibits beautiful polarization colors, and generally an evident twinning striation in some 

 of its grains. Sometimes it shows a banding of different colors between crossed Nicols. It is cut 

 by innumerable irregular cracks, by which finally impurities enter and change its average compo- 

 sition. The augite is apt to be somewhat fibrous from incipient decay, but when fresh its play of 



*Owen's geological report of Wisconsin, Iowa and Minnesota, p. 360. 



