704 THE GEOLOGY OF MINNESOTA. 



[Green wacke. 



Meg. Of the lot thin sections have been made of only four. They are separately 

 distinguishable for purposes of description as (), (V), (c) and (d), though these 

 distinctions were not recorded in the field-book. 



(a) Macroscopically, the sample has a variable character, a fine-grained, dark 

 rock, being associated with a coarser one, both being dark gray or greenish. 



Mic. The section was evidently made from the coarser sort. The section 

 consists essentially of long, spindling feldspars, somewhat radiating in arrangement 

 and of amphibole, with some small, scattering grains of pyrite and of magnetite. In 

 polarized light some quartz grains are brought to light. The materials of the rock 

 are probably all, except the quartz, of eruptive, basic origin, and the structure of the 

 slide indicates a fragment of an altered diabase. The intimate relations of the feld- 

 spar lamellae with the amphibole, give the impression that they are both of 

 secondary origin. Sometimes a rod of amphibole separates two otherwise contiguous 

 lamellae of the feldspar, and on emerging from the feldspar the rod of amphibole 

 spreads out into a fan-shaped or palmate series of fibres. In most cases, however, 

 such rods terminate near the termination of the feldspar lamellae. The amphibole 

 does not show any characteristic cleavages, but rather a fibrous and fragmental 

 structure. (Compare the comes vertes of Michel Levy. Comptes Rendus, Societe 

 Geologique de France; Sess Extrord. 1890, page 916.) One section. 



(b) Meg. Is a light-gray, fine-grained, apparently siliceous or feldspathic rock, 

 having a somewhat elongated or schistose structure. 



Mic. In high powers the section shows principally a greenish yellow (pyroxenic 

 or amphibolic) element constituting at least one-half of the rock. It polarizes like 

 pyroxene, probably diopside. Mingled with these is what appears to be granular 

 leucoxene or sphene, but as it is in groups and isolated it is more like the former. 

 According to Prof. Lacroix, the piece is almost certainly an altered limestone, what 

 the French petrographers call corne verte. 



In the midst of the green element is a granular transparent mineral which has 

 the aspect of a feldspar and which on making the following tests proves to be 

 anorthite. 



I made a powder of the rock, and boiled the powder in hydrochloric acid for 

 fifteen or twenty minutes. After washing and coloring it with malachite green, 

 and again washing, the powder remained permanently colored, i. e., a large portion 

 of it did, showing an attack by the acid and the formation of a gelatinous or skeleton 

 silica, like that formed by anorthite under such treatment. Yet in this colored 

 powder are still numerous, clear, nearly colorless grains, which have high refraction 

 and high double refraction. While clear and glassy they still have a little tinting of 

 a yellow or a greenish yellow. They constitute in some cases apparently one-third 



