254 THE GEOLOGY OP MINNESOTA. 



[Diabase. 







ferrite, but the copper disappears when the alteration is carried still further, as it 

 often is in the section. The feldspar here is largely replaced by viridite and 

 chalcedonic patches. In the sections of the more highly altered portions of this 

 rock, the augite is changed, for the most part, to a dirty green viridite and chlorite, 

 which show oftentimes a spherulitic structure. In the more highly altered parts 

 many apatite needles occur. 



" In the portions of the rock which are most altered, the chalcedony forms 

 beautifully polarizing radiating concretions, and the general appearance of the rock 

 is that of granite instead of that which it really is an altered, coarse-grained 

 diabase." 



A Boricky test of the pinkish feldspar gave only lime and soda, the former being 

 in large crystalliths of monoclinic form, and the latter as small hexagonal rods. No 

 signs of potash were observed, and the feldspar may hence be assigned to the 

 prevalent category, viz., lalradorite. 



The mineral, however, which seems to be that called chalcedony, since it is 

 radiated in structure and polarizes in brilliant radiating colors, is certainly a biaxial 

 mineral, and hence cannot be any form of quartz. Quartz, when of the same thick- 

 ness, would show the same color in all positions and directions when its axis is 

 parallel to the section as in the radiating fibres, and also when parallel to either 

 nicol it would simply be dark. A single test in convergent light confirms this, since 

 the interference figure is either a single optic axis or a curved black bar of a biaxial 

 hyperbola, or occasionally a bisectrix (X). The fibres which are cut perpendicular 

 to their longer dimensions show sometimes rectangular shapes. Such forms are 

 found when the centre of a spherule or a radiated mass is cut, and in such case the 

 highest colors of double refraction are found; and such sections show the figure of 

 the optic normal (,) in convergent light, indicating the axial plane is transverse 

 to the longer dimensions of the fibres. In other words, the mineral is orthorhombic 

 or tetragonal and has a high double refraction, and its elongation is perpendicular 

 to the axial plane. Further, when the fibres are cut parallel to their elongation, 

 they extinguish parallel. The bisectrix n r is in the acute angle of the optic axes. 

 Measured in air by the axial goniometer of Lacroix (American Geologist, February, 

 1896) this angle is found to be about 50. 



These characters, combined as they are, can only be found in thontsoitite, which 

 is not a very uncommon mineral in the coarse traps of the region, though generally 

 in amygdules of much larger size. This radiated mineral being excluded from that 

 category, there seems to be no free quartz in the sections examined. 



Four sections examined, three being the same as examined by Dr. Wadsworth. 



Aye. Cabotian. 



