392 THE GEOLOGY OP MINNESOTA. 



[Forellenstein. 



polarizer and condenser. They are distinguishable from the olivine by reason of 

 their clear white color in parallel light, their marked, single cleavage, which is 

 parallel with one nicol at the instant of extinction, and by their very low double 

 refraction, even in very thick section. 



In order to be satisfied that this mineral is really olivine, further examination 

 was made, viz.: 



1. Another thin section was prepared, and about one-half of it was not covered 

 by the glass lamella, 



2. This uncovered portion was immersed in nitric acid for eighteen hours. 

 There not being sufficient evidence of solution, 



3. This uncovered portion was kept for five hours in a strong hydrochloric 

 acid at 40 centigrade. In order to detect any gelatinous silica, 



4. A part of the slide thus exposed to acids was covered with an aniline color, 

 and then after thorough washing and even long soaking in clean water, 



5. On examination there was a characteristic stain of the aniline in some of 

 the border areas about the magnetite. But as there is very little of olivine in the 

 portion thus treated, but little permanent coloration was produced. The other 

 minerals were not visibly affected by these acids. 



It seems plain, hence, that there is an olivine later than the feldspars. There 

 is also an earlier olivine, which is now so changed that it takes the form of an 

 isotrope, and must be considered to date from the first consolidation. The later 

 olivine probably dates from the epoch of cooling, as already suggested for the rock 

 No. 1. 



Remark. Considering its exposed position this rock is little weathered. The 

 relation of the olivine to the feldspar, as seen in No. 258, is illustrated by figure 18. 



N. H. w. 

 No. 513. FORELLENSTEIN. 



From an outcrop near Duluth; N. W. % N. E. J sec. 27, T. 50-15; at about the same altitude as No. 512, 

 and not far distant. 



Ref. Annual Report, x, page 35; Bulletin ii, page 95. 



This rock consists, as remarked by Wadsworth, essentially of plagioclase and 

 olivine, making a forellenstein. The sections are too thick to be of use in making 

 any further determinations. The rock is well preserved. It is probable that the 

 lateness of the last glacial period has not allowed sufficient time for much decay on 

 these summits, since the removal of the pre-glacial decay. 



In another section of this rock appears a little dial/age, surrounding, or at least 

 embracing, the early olivines, accompanied by a little inayiu'titesiud still less of biotite. 



Three sections. 



Age. Cabotian, 



