242 THE GEOLOGY OP MINNESOTA. 



[Laumontite. Shale. 



I injite appears in two generations. The older generation is represented by 

 crystals of idiomorphic outlines, which preceded the feldspars, and which sometimes 

 enclosed small quantities of the magma. The second generation is represented by 

 a multitude of minute idiomorphic crystals, which lie within the nearly isotropic 

 residuum. 



The olivines are altered, for the most part, to bowlingite. 



There are a few green and almost isotropic areas, but occasionally they show a 

 fibrous structure, which allies them with the fibrous xi/jimittc (thalite) mentioned in 

 No. 162. 



In the feldspars is also a light green alteration mineral presumed to be chlorite. 

 It shows a slight absorption. Other grains included in the feldspars, polarizing 

 brightly, are probably mica. 



Three sections. 



Age. Manitou (?) 



Remark. There is a substance (glass), which, while not abundant, serves as a 

 matrix for all the other materials. It might at first glance be taken for augite, since 

 in natural light it has a brown color, but it is entirely destitute of cleavage, embraces 

 opaque ferruginous impurities and remains dark on rotation between the nicols. 

 That it is yet a translucent glass indicates the fixedness of mineral composition 

 through the long history which the rock has suffered, and the necessity of attribut- 

 ing the prevalent alteration seen in these trap rocks below the natural surface to 

 some other cause than ordinary weathering and length of exposure. It is allowable 

 to refer the most of this alteration to gases and hot waters that penetrated the mass 

 before it was cooled, after solidification. N. H. w. 



No. 176A. LAUMONTITE. 



This number is not mentioned in the Ninth Annual Report, page 47, but it evidently represents the 

 material of the red joints in No. 176. 



Meg. Sheets of red to white laumontite, with some calcite. The section marked 

 No. 176A was made from one of the rock fragments attached to the laumontite. This 

 section is similar to No. 176, but more altered. 



Age. Manitou. u. s. G. 



No. 177. SHALE (with zeolitic nmi/i/dd/oiiij. 



Underlies No. 176. A bed eight feet thick. See No. 626. 



Ref. Annual Report, ix, pages 47, 48; Annual Report, x, page 60. 



Meg. There are two hand samples, one of the amygdaloid and the other of the 

 shale. The former is a very fine-grained, almost aphanitic, rock, dark brown or 

 almost black in color. It is thickly strewn with small amygdules of laumontite. 



