204 THE GEOLOGY OF MINNESOTA. 



[tMabase. 



plate (Table des Birefringences) of the same volume, and is evidently greater than 

 .08 millimeters. Now this pyroxene mineral gives, as its highest interference color, 

 at this thickness a color green. But this must be the green of the third order since 

 there are two violet tints visible below it, one on the surface of the thin section, and 

 one on the oblique edges. 



Foraugite n, ,,, = 0.01(>. n, u .,.= 0.006. n t n v = 0.022. 



For enstatite , ", 0.005. n,,, - P = 0.004. n, - n,, - 0.009. 



From this it appears that, making a little allowance for the obliquity of the 

 thin section, the value 0.016 would agree well with a green in the third order for 

 augite in a section of the thickness of over O.OS millimeters, while for enstatite, at 

 0.005, it would be impossible to produce a green of the third order in a section of this 

 thickness, perpendicular to , if at all. 



A test by the Boricky method of microchemical examination showed many 

 short minute forms taken for fluosilicate of lime, and flattened elongated rhombohedra 

 that result from the presence of magnesia. This result is not conclusive, owing to 

 the resemblance of the micro-crystals of lime in this test, to those of magnesia, and 

 hence the possibility of both these forms coming from magnesia of an enstatite. Still 

 both lime and magnesia are strongly indicated. 



In order to be still more sure of this conclusion, the rock was powdered. On 

 attempting to separate the pyroxene by iodide of methyl it was found that the specific 

 gravity of the mineral is so near that of the iodide of methyl that while most of the 

 feldspar floated, some of it was carried down with the pyroxene and magnetite. 

 Indeed, on examining with a. loop the powder consisting largely of pyroxene, it was 

 seen that not only had the pyroxene carried down small quantities of the labradorite, 

 but some magnetite had also acted in the same way, and that hence the separated 

 pyroxene powder was not pure. However, eight or ten small grains were mechan- 

 ically separated from the powder. These were entirely free from labradorite, and. 

 their size was about one-fourth to one-tenth the size of a pinhead. 



These grains were dissolved in hydrofluoric acid, with added sulphuric, and 

 evaporated to dryness. The residue was dissolved in water with a few drops of hydro- 

 chloric acid, and on slow evaporation on a glass slide, under the microscope, minute 

 spangles and radiated clusters of gypsum were formed, indicating the presence of lime. 



With another portion of the same residue a further test was made, viz., dissolved 

 in hydrochloric acid, and after adding chlorhydrate of ammonia and ammonia the 

 resulting precipitate (iron and alumina) was filtered away. The filtrate was tested 

 for lime by adding oxalate of ammonia, which also gave a copious precipitate, again 

 indicating lime. The presence of lime differentiates this mineral from enstatite, and 

 with the other characters shows it is really augite. 



