458 THE GEOLOGY OP MINNESOTA. 



[Amygdaloid and tuff. 



two of scolescite and a pebble of lintonite, the last being in the centre. In the fine, 

 short-fibred lintonite the highest color, being that presented by sections parallel to 

 the axial plane, compared with the color given by barite in the same position, showed 

 that the double refraction of lintonite is about 0.017 to 0.018, that of thomsonite being 

 0.028. A comparison with scolescite cut in the same position gave the same result. 

 The single refraction of lintonite is also less than that of thomsonite. Its specific 

 gravity is 2.372. 



In a thin section of lintonite the negative fibres are more light than the positive, 

 the latter being nearly dark. Indeed, the negative fibres are nearly as highly 

 doubly refractive as those which present ,, and can be > distinguished from them 

 only by their shapes, the n m sections being squarish. Five sections. 



Aye, Manitou. N. H. w. 



No. 626. AMYGDALOID AND TUFF. (&ubma/ri/ne.) 



About midway between Poplar and Temperance rivers (Nos. 177 and 178). This lies in regular beds of a 

 few inches, or less than an inch, like a sedimentary rock. The specimens may represent the red amygdaloid of 

 several miles along here, though the sedimentation is not so plain in all cases. Over these layers is a heavy bed 

 of trap, massive or coarsely jointed, which forms arched purgatories and tables extending into the lake, the 

 amygdaloid being eaten out by the friction of the beach line under the action of the waves. 



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



Meg. Pinkish, red, or brownish, specked with laumontite, and sometimes 

 largely made up of secondary minerals, frequently zeolitic. Evidently consisting 

 principally of eruptive matter originally, perhaps in the form of coarse ash, but 

 distributed by water. Some parts, however, are completely pumiceous, the cavities 

 filled now by laumontite. Fragmental, red, shaly matter is mingled confusedly more 

 or less in these layers. Probably represents the so-called "ash-bed diabase." 



NIC. The fine, rusty, groundmass is crowded with micolitic plagioclases. In 

 many spaces, which appear to have been occupied originally by porphyritic feldspars, 

 are grains of laumontite whose optic axial angle (2 E) is quite small, apparently not 

 over 10, which causes the interference figure in convergent light to present, on 

 rotation, a deceptive approach to the manner of revolution of a unaxial interference 

 figure. The axial plane is parallel with the fibration, and consequently some of the 

 grains show the bisectrix w p and some n m , while the most of them are cut intermediate 

 between those axes; and the elongation is hence continually positive. 



There is also a sparse representation of what appears to be ay//m/r///V mineral. 

 The section being of a thickness of about .035 millimeter, the highest polarization 

 color of this mineral is greenish blue, the fringes running downward through red 

 and yellow, thus indicating augite. It is a remarkable illustration of the preservation 

 of that mineral in the same mass where the original feldspars are wholly replaced 

 by a zeolite. One section. 



Age. Manitou. 



